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HOW TO… Engineering Guide

A Simple Substation Grounding Grid Analysis

Using Autogrid Pro

2017 Release

Page iv

REVISION RECORD

Date Version Number Revision Level

January 2001 9 0

November 2002 10 0

June 2004 11 0

December 2006 13 0

January 2012 14 0

June 2017 16 0

Address comments concerning this manual to:

Safe Engineering Services & technologies ltd.

___________________________________________

3055 Blvd. Des Oiseaux, Laval, Québec, Canada, H7L 6E8

Tel.: (450) 622-5000 FAX:(450) 622-5053

Email: [email protected]

Web Site: www.sestech.com

Copyright 2000-2017 Safe Engineering Services & technologies ltd. All rights reserved.

SPECIAL NOTE

As SES software is constantly evolving, with frequently created updates, minor

discrepancies may appear between this How To manual illustrations of the

software interface and the present software version interface. These differences

are cosmetic in nature and do not impact the validity of the guidance and

procedures provided herein. Furthermore, small differences in the reported and

plotted numerical values may exist due to continuous enhancements of the

computation algorithms.

TABLE OF CONTENTS

Page

Page vii

CCCHHHAAAPPPTTTEEERRR 111 INTRODUCTION ................................................................................................. 1-1

1.1 OBJECTIVE ........................................................................................................................... 1-1

1.2 GROUNDING PROBLEM ...................................................................................................... 1-2

1.3 COMPUTER MODELLING TOOL ......................................................................................... 1-2

1.4 METHODOLOGY OF THE GROUNDING DESIGN .............................................................. 1-2

1.5 ORGANIZATION OF THE MANUAL ..................................................................................... 1-3

1.6 SOFTWARE NOTE ................................................................................................................ 1-4

1.7 FILE NAMING CONVENTIONS ............................................................................................ 1-4

1.8 DEMO EVALUATION ............................................................................................................ 1-6

1.9 WORKING DIRECTORY ....................................................................................................... 1-6

1.10 INPUT AND OUTPUT FILES USED IN TUTORIAL ................................................. 1-6

CCCHHHAAAPPPTTTEEERRR 222 DESCRIPTION OF THE PROBLEM & DEFINITION OF THE SYSTEM ............ 2-1

2.1 THE SUBSTATION GROUNDING SYSTEM ........................................................................ 2-1

2.2 THE OVERHEAD TRANSMISSION LINE NETWORK ......................................................... 2-2

2.3 THE SUBSTATION TERMINALS .......................................................................................... 2-3

2.4 THE SOIL CHARACTERISTICS ........................................................................................... 2-3

CCCHHHAAAPPPTTTEEERRR 333 PROGRAM HIGHLIGHTS & USING AUTOGRID PRO ...................................... 3-1

3.1 PROGRAM HIGHLIGHTS ..................................................................................................... 3-1

3.2 USING AUTOGRID PRO ....................................................................................................... 3-2

3.2.1 STARTING AUTOGRID PRO ................................................................................... 3-2

3.2.2 WORKING WITH PROJECTS AND SCENARIOS ................................................... 3-3

3.2.3 SPECIFYING DATA FOR A SCENARIO .................................................................. 3-4

3.2.4 PROCESSING A SCENARIO ................................................................................... 3-5

3.2.5 ADDING NEW SCENARIOS ..................................................................................... 3-5

3.2.6 CLOSING A PROJECT ............................................................................................. 3-6

3.2.7 ENDING YOUR AUTOGRID PRO SESSION ........................................................... 3-6

CCCHHHAAAPPPTTTEEERRR 444 CREATIONG A PROJECT AND SCENARIO ..................................................... 4-1

4.1 START-UP PROCEDURES ................................................................................................... 4-1

4.1.1 CREATING A NEW PROJECT ................................................................................. 4-6

4.1.2 OPENING AN EXISTING PROJECT ........................................................................ 4-8

4.1.3 USING THE PROJECT ............................................................................................. 4-8

TABLE OF CONTENTS (CONT’D)

Page

Page viii

4.1.4 FILES THAT ARE PART OF THE PROJECT ........................................................... 4-9

CCCHHHAAAPPPTTTEEERRR 555 SOIL RESISTIVITY DATA ENTRY .................................................................... 5-1

5.1 A HORIZONTAL TWO-LAYER SOIL MODEL ...................................................................... 5-1

5.2 SOIL RESISTIVITY DATA ENTRY ........................................................................................ 5-2

CCCHHHAAAPPPTTTEEERRR 666 INITIAL GROUNDING GRID DESIGN ............................................................... 6-1

6.1 DATA ENTRY ......................................................................................................................... 6-1

CCCHHHAAAPPPTTTEEERRR 777 FAULT CURRENT DISTRIBUTION ANALYSIS ................................................ 7-1

7.1 INTRODUCTION .................................................................................................................... 7-1

7.2 PREPARATION OF THE INPUT DATA ................................................................................ 7-2

7.2.1 DATA ENTRY ............................................................................................................ 7-3

CCCHHHAAAPPPTTTEEERRR 888 PERFORMANCE EVALUATION OF EAST CENTRAL SUBSTATION ............ 8-1

8.1 SAFETY CRITERIA ................................................................................................................ 8-1

8.1.1 TOUCH VOLTAGES ................................................................................................. 8-2

8.1.2 STEP VOLTAGES ..................................................................................................... 8-2

8.1.3 GPR MAGNITUDE .................................................................................................... 8-3

8.1.4 GPR DIFFERENTIALS .............................................................................................. 8-3

8.1.5 DETERMINING SAFE TOUCH AND STEP VOLTAGE LEVELS ............................. 8-3

8.1.6 A SIMPLER WAY TO SPECIFY THE LOCATION OF OBSERVATION POINTS .... 8-5

8.2 PLOTS AND REPORTS ........................................................................................................ 8-5

8.2.1 SELECTING PLOTS AND REPORTS ...................................................................... 8-5

8.2.2 CUSTOMIZING PLOTS ............................................................................................. 8-8

8.2.3 CARRYING OUT THE COMPUTATIONS AND PRODUCING THE PLOTS AND REPORTS ................................................................................................................. 8-8

8.3 ANALYSIS OF THE RESULTS ........................................................................................... 8-10

8.3.1 USING THE GRAREP UTILITY .............................................................................. 8-10

8.3.2 GENERAL INFORMATION REPORTS ................................................................... 8-11

8.3.3 SOIL RESISTIVITY ANALYSIS ............................................................................... 8-13

8.3.4 GROUND GRID PERFORMANCE AND SAFETY ANALYSIS ............................... 8-15

8.3.5 COMPUTATION OF FAULT CURRENT DISTRIBUTION ...................................... 8-21

CCCHHHAAAPPPTTTEEERRR 999 REINFORCING THE GROUNDING SYSTEM ................................................... 9-1

9.1 EXPONENTIAL GRID DESIGN ............................................................................................. 9-1

TABLE OF CONTENTS (CONT’D)

Page

Page ix

9.1.1 CREATING THE EXPONENTIAL GRID SCENARIO ............................................... 9-2

9.1.2 OPENING THE EXPONENTIAL GRID SCENARIO ................................................. 9-2

9.1.3 MODIFYING THE EXPONENTIAL GRID SCENARIO ............................................. 9-2

9.2 ADDING GROUND RODS ..................................................................................................... 9-6

9.2.1 THE DETAILS ........................................................................................................... 9-6

9.3 EXPORT GROUNDING GRID INTO DXF FILE .................................................................... 9-8

CCCHHHAAAPPPTTTEEERRR 111000 USING GRSERVER .......................................................................................... 10-1

10.1 STARTING GRSERVER ......................................................................................... 10-1

10.2 CREATING 3D PLOTS ........................................................................................... 10-2

10.3 CREATING 2D PLOTS ........................................................................................... 10-5

10.4 SAVING AND PRINTING PLOTS .......................................................................... 10-5

10.5 SUMMARY .............................................................................................................. 10-6

CCCHHHAAAPPPTTTEEERRR 111111 CONCLUSION .................................................................................................. 11-1

Chapter 1. Introduction

Page 1-1

CCCHHHAAAPPPTTTEEERRR 111

INTRODUCTION

1.1 OBJECTIVE

This How To… Engineering Guide shows

you how to carry out a typical substation

grounding design using the AutoGrid Pro

software package. The AutoGrid Pro

package combines the computational

power of the computation modules

RESAP, MALT and FCDIST of the

CDEGS software package with a simple,

largely automated interface. The result is

an-easy-to use, yet powerful, grounding

analysis program. A step-by-step approach

is used to illustrate how to use the program

to input your data, carry out the

computation and explore the computation

results.

Please note that you may press the F1 key at

any time to display context-sensitive on-

line help pertinent to the topic to which you

have given focus with your mouse. You

may also access the complete help file by

selecting Contents from the Help menu of

the main AutoGrid Pro interface.

If you are anxious to start entering data and

running AutoGrid Pro you may do so by

reading Section 1.5 of this chapter and

skipping the rest of this chapter and Chapter

2. We strongly recommend, however, that

you refer to the skipped sections to clarify

items related to input files, system configuration and data, file sharing and design methodology.

Please call SES’ toll-free support line with any questions you may have, as you work through

this manual. Call us collect at +1-450-622-5000 if you do not have this number handy. You can

also E-mail us questions at [email protected].

mailto:[email protected]


Page 1-2

1.2 GROUNDING PROBLEM

The grounding analysis problem

discussed in this manual is

illustrated in Figure 1.1. A new 230

kV Substation (named East

Central) is planned. It will be

interconnected to the rest of the

network via three transmission

lines terminating at three different substations, namely Terminals Greenbay, Newhaven and Hudson

respectively. The objective of the analysis is to provide a new grid design for East Central Substation.

The final design is to limit touch and step voltages to safe levels for personnel within the substation

area, based on up-to-date system data, appropriate measurement techniques and instrumentation, and

state-of-the-art computer modeling methods.

1.3 COMPUTER MODELLING TOOL

SES’ AutoGrid Pro is used to model the field measurements (i.e., soil resistivities and grounding

system impedance) and interpret the measured data, to compute the distribution of fault current

between the transmission line static wires, distribution line neutral wires, and the substation grounding

grid, and to simulate a representative phase-to-ground fault in the substation in order to compute the

ground potential rise and ground resistance, touch voltages, step voltages, and earth potentials

throughout the substation.

This software integrates all the tools required for such an analysis. It includes:

A soil resistivity analysis module to determine the soil structure from soil resistivity

measurements.

A fault current distribution analysis module to compute the fraction of the fault current that is

discharged in the grounding grid.

A grounding module that computes the response of the grounding grid to the fault current.

A safety analysis module that computes the touch and step potentials above the grid and

compares them to safety limits deduced from the relevant standards.

The results are presented in graphical and tabular form; several detailed reports are available.

1.4 METHODOLOGY OF THE GROUNDING DESIGN

A grounding design analysis is normally carried out in six major steps as follows:

Step 1 The first step of the study is aimed at determining a soil model that is equivalent to the real

earth structure. This is done using the soil resistivity analysis module, RESAP. Any of several

East CentralSubstation

GreenbayTerminal

HudsonTerminal

NewhavenTerminal


Page 1-3

soil type models can be selected by the design engineer as an approximation to the real soil

(uniform, two-layer, multilayer, etc.).

Step 2 Based on experience and on the substation ground bonding requirements, a preliminary

grounding system configuration is developed and a simulation is carried out (initial design).

Step 3 The configuration and characteristics of the transmission lines connecting this substation to

adjacent substations are defined. This allows the program to determine what fraction of the

total fault current actually flows into the grounding grid of the studied substation.

Step 4 The calculated results are analyzed and various computation plots and printout reports are

examined to determine if all design requirements are met. In particular, the safe touch and step

voltage thresholds are determined, based on the applicable standards and regulations, and are

compared to the computed values.

Step 5 If not all design requirements are met or if all these requirements are exceeded by a

considerable margin, suggesting possible significant savings, design modifications to the

grounding system or to the transmission line network are made and the design analysis is

restarted. This normally involves carrying out Step 2, then Steps 4 and 5.

Step 6 If seasonal soil resistivity variations must be taken into account, then the entire analysis is

repeated for every realistic soil scenario and the worst-case scenario is used to develop the final

design.

1.5 ORGANIZATION OF THE MANUAL

In accordance with the design methodology described above, the manual is organized as follows:

Chapter 2 outlines the problem being modeled and defines the system data required for the study.

Chapter 3 briefly introduces the components of the AutoGrid Pro program and also describes in general

how to work with Projects and Scenarios in AutoGrid Pro.

Chapter 4 shows how to get started with the program by creating a project and first scenario.

Chapter 5 describes the data entry for the soil measurements module (RESAP), which is used to

interpret the soil resistivity data based on measurements taken at East Central Substation (Step 1).

Chapter 6 presents the initial design of the grounding system. It describes in detail how to use AutoGrid

Pro to set up the initial design of the grounding grid at East Central Substation (Step 2).

Chapter 7 describes data entry for the fault current distribution module (FCDIST), which is used to

determine the fault current distribution (for the fault current simulations) between the transmission

line static wires, distribution line neutral wires, and the substation grounding grid (Step 3).

Chapter 8 presents the ANSI/IEEE safety criteria applicable to substation grounding. The fault

simulation results are presented in graphical and report formats. Grid potential (GPR), touch voltages,

and step voltages are provided in detail (Step 4).


Page 1-4

Chapter 9 presents the design of the reinforced grounding system. It describes how you can easily

repeat the computations from Chapters 6 and 7 to meet the safety criteria (Step 5).

Chapter 10 shows how to use the GRServer program to examine the computation results of AutoGrid

Pro in greater detail.

In Chapter 11, the conclusions of the study are summarized. Step 6 is not considered in this manual.

1.6 SOFTWARE NOTE

Depending upon your software license terms, some of the options described in this document may not

be available to you. When this is the case, a lock symbol will be displayed next to the unavailable

options in the user-interface screens.

1.7 FILE NAMING CONVENTIONS

It is important to know which input and output files are created by the CDEGS software. All CDEGS

input and output files have the following naming convention:

XY_JobID.Fnn

where XY is a two-letter abbreviation corresponding to the name of the program which created the file

or which will read the file as input. The JobID consists of string of characters and numbers that is used

to label all the files produced during a given CDEGS run. This helps identify the corresponding input,

computation, results and plot files. The nn are two digits used in the extension to indicate the type of

file.

The abbreviations used for the various CDEGS modules are as follows:

Application Abbreviation Application Abbreviation

RESAP RS FCDIST FC

MALT MT HIFREQ HI

MALZ MZ FFTSES FT

TRALIN TR SESEnviroPlus TR

SPLITS SP SESShield-3D SD

SESTLC TC ROWCAD RC

SESShield LS SESeBundle BE

GRSPLITS-3D SP CorrCAD CC

AutoGroundDesign AD SESThreshold TH

SESAmpacity AP SESCrossSection XS

SESImpedance FM CSIRPS* CS

* The CSIRPS module is used internally by the graphics and report generating interfaces.

The following four types of files are often used and discussed when a user requests technical support

for the software:


Page 1-5

.F05 Command input file (for computation applications programs). This is a text file that can

be opened by any text editor (WordPad or Notepad) and can be modified manually by

experienced users.

.F09 Computation results file (for computation applications programs). This is a text file that

can be opened by any text editor (WordPad or Notepad).

.F21 Computation database file (for computation applications programs). This is a binary file

that can only be loaded by the CDEGS software for reports and graphics display.

.F33 Computation database file (for computation applications programs MALZ and HIFREQ

only). This is a binary file that stores the current distribution to recover.

For further details on CDEGS file naming conventions and JobID, consult the CDEGS Help by

pressing F1 in the main CDEGS interface and navigating to Using CDEGS – Working With CDEGS

Projects – File Naming Conventions.

In CDEGS-Legacy, the same help entry is available under the menu Help | Contents | File Naming

Conventions.


Page 1-6

1.8 DEMO EVALUATION

In order to be able to evaluate SES Software without a license, you should install the software as a

demo. This will give you access to the computed results without extra effort.

In the demo environment, the input and output files of the case studies in this tutorial are already

installed under the SES Software documents subfolder, HowTo; e.g.,

“C:\Users\Public\Documents\SES Software\<version>\HowTo\Autogrid Pro”, where <version> is the

version number of SES Software. You must use this default location as the working directory when

the software is installed as a demo.

1.9 WORKING DIRECTORY

A Working Directory is a folder where all input and output files of case studies are stored and created.

In this tutorial, we recommend storing the working directory of the tutorial under the following folder:

<drive>\CDEGS HowTo\Autogrid Pro

e.g., C:\CDEGS HowTo\Autogrid Pro

You may prefer to use the default location offered by the demo installation of SES Software or another

location altogether, but, either way, you should take note of the full path of the working directory

before running Autogrid Pro, as you will need this information to follow this tutorial.

1.10 INPUT AND OUTPUT FILES USED IN TUTORIAL

All input and output files used in this tutorial are supplied from the SES Software distribution. When

the software is installed as a demo, the full set of distribution files are unpacked from the package file

and stored under the default SES Software documents subfolder, Setup.Z, where “Z” is part of the

version number of the software. Note that the package file, SESXY.EXE, may be unpacked at any

time (“X” and “Y” are part of the version number of the software) if the tutorial is being followed

without a demo installation. The required input and output files of this tutorial can be found in the

folders listed below in the distribution.

If you are constructing your own working directory or you would like to refresh your files, then you

can manually copy the original files in the distribution from the following subfolders:

Input Files: Examples\Official\HowTo\Autogrid Pro\inputs

Output Files: Examples\Official\HowTo\Autogrid Pro\outputs

Note that the files found in both the ‘inputs’ and the ‘outputs’ subfolders should be copied directly

into the working directory, not into subdirectories.


Page 1-7

After the tutorial has been completed, you may wish to explore the other how-to engineering manuals;

they can be accessed from the program shortcut, SES Software X.Y > Documentation > Manual.

The same manuals can also be retrieved from the SES Software distribution under the subfolder,

“PDF\HowTo Manuals”.

Chapter 2. Description of the Problem & Definition of the System Data

Page 2-1


DESCRIPTION OF THE PROBLEM &

DEFINITION OF THE SYSTEM

The system being modeled is located in an isolated area (i.e., not in an urban area and not close to any

pipelines), where there are no major

geological disturbances (ocean,

rivers, valleys, hills, etc.). It consists

of the following three major

components (see Figure 2.1):

1. The substation and associated

grounding system of the substation

under study;

2. An overhead transmission line

network;

3. Various substations (terminals)

from which power is fed to the

transmission line network.

Soil resistivity measurements have

been carried out at the substation site

under study and are available.

Figure 2.1 Schematic of System under Study

2.1 THE SUBSTATION GROUNDING SYSTEM

Figure 2.2 shows the configuration of the initial design of the East Central grounding grid, which

consists of a 100 m by 60 m (328 feet by 197 feet) rectangular grid buried at a depth of 0.5 m (1.64

feet). Each conductor has a radius of 0.6 cm (0.02 feet or 0.23"): these are 4/0 copper conductors.

There are 9 equally spaced conductors along the X axis and 7 equally spaced conductors along the Y

axis. The perimeter of the grid was defined such that the outermost conductors are located 1 m ( 3.3

ft) outside the edge of the fence to protect people standing outside the substation from excessive touch

voltages. The fence is regularly connected to the outermost conductors. The fence posts, however,

(which are metallic) have been omitted for simplicity. It is an easy task to add the fence posts using

the “Create Rods” tool in AutoGrid Pro, as explained later. The initial ground resistance of the East

Central Substation is 0.538 as will be determined by the MALT computation module.

Greenbay

(R = 0.2 )

Terminal

g A

Hudson

(R = 0.3 )

Terminal

g A

Newhaven

(R = 0.3 )

Terminal

g A

East Central

(Re to be computed)Substation

230 kV

230 kV

230

kV

(6.8

mile

s or

11

km)


Page 2-2

Figure 2.2 Initial Design of the Grounding System at the East Central Substation

Note that more complex grid shapes can easily be created: conductors may be modeled in any 3-

dimensional orientations.

2.2 THE OVERHEAD TRANSMISSION LINE NETWORK

There are three double-circuit transmission lines

leaving the East Central substation. The average

span length of the transmission lines is 330 m

(1083'). The first transmission line is 64 spans

long (21 km or 13 miles) and is connected to the

Greenbay substation (terminal). Another

transmission line is 33 spans long (11 km or 6.8

miles) and is connected to the Newhaven

substation. The remaining transmission line is

connected to the Hudson substation and is 25

spans long (8.3 km or 5.2 miles). Each tower has

two 7 No. 8 Alumoweld type shield wires and

the phase wires are 795 MCM Drake. The GMR

and the average DC resistance of the shield

wires are 0.00064 m (0.0021 feet) and 1.76

/km (2.83 /mile), respectively. Figure 2.3

shows a cross section of the transmission line

used in this study.

The ground resistances of the transmission line

towers in the Greenbay - East Central arm of the

network, which are 330 m (1083') apart, are all

estimated to be equal to 10 . The towers in the

Hudson - East Central and Newhaven - East

Central arms, which are also 330 m apart, have

a higher estimated resistance of 28

Figure 2.3 Transmission Line Configuration

(0,0,0.5)

60 m

100 m(100,60,0.5)

Y

X


Page 2-3

2.3 THE SUBSTATION TERMINALS

The ground resistances of the terminals are

equal to 0.2 , 0.3 and 0.3 for Greenbay,

Hudson and Newhaven terminals,

respectively. Figure 2.4 illustrates a circuit

diagram of the power system under study

during a phase-to-ground fault on Phase B2

at East Central Substation.

Figure 2.4 Power System Network Analyzed in Example

In this study, we assume that the highest fault current discharged into the earth by the East Central

Substation grid occurs for a 230 kV single-phase-to-ground fault at East Central Substation on Phase

B2 of Circuit 21. Let us suppose that short-circuit calculations carried out by the power utility provide

the following fault current contributions from Phase B2 of each terminal substation for a fault at East

Central:

Greenbay: 1226 - j 5013 A

Hudson: 722 - j 6453 A

Newhaven: 745 - j 5679 A

2.4 THE SOIL CHARACTERISTICS

Detailed soil resistivity measurements have been carried out at the substation site, using the Wenner

4-pin technique (i.e., the distances between adjacent electrodes are equal). Table 2-1 gives the apparent

resistance values measured at the substation site. Note the exponentially increasing pin spacings and

extent of the largest spacings. This is of capital importance to achieve a reliable grounding grid design.

In fact, usually more than one set of measurements are made in different directions and at different

locations throughout the substation site, as well. Each set of measurements is then interpreted

independently.

1 Note that it is usually conservative to model a fault occurring on the phase furthest from the static wires, since this results

in the lowest current pulled away from the substation grounding grid by means of magnetic field induction between the

faulted phase and the static wires. Other scenarios can of course be investigated with the software.


Page 2-4

Separation Depth of Depth of Apparent

Between

Adjacent

Probes1

Current

Probes2

Potential

Probes2

Resistance

(V/I)

(meters) (meters) (meters) (ohms)

0.3 0.1 0.05 152.300

1 0.1 0.05 48.160

2 0.1 0.05 6.120

5 0.1 0.05 3.340

7 0.15 0.05 1.760

10 0.15 0.05 1.110

15 0.3 0.05 0.692

25 0.3 0.05 0.441

35 0.3 0.05 0.320

50 0.6 0.1 0.218

65 0.6 0.1 0.156

90 0.6 0.1 0.106

120 1 0.1 0.079

150 1 0.1 0.064

Table 2-1 Apparent Resistances Measured at Substation Site Using the Wenner Method

1 Also known as the “a” spacing associated with the Wenner technique.

2 These values are used to determine soil resistivities close to the surface with better accuracy. Knowing these values is

therefore important only for the first few pin spacings. At larger spacings, as a practical matter, the current probes should

be driven deeper in order to increase the strength of the signal measured between two potential probes.

Chapter 3. Program Highlights & Using Autogrid Pro

Page 3-1


PROGRAM HIGHLIGHTS & USING

AUTOGRID PRO

In this chapter, we will briefly describe the highlights and major functions of the program. A more

detailed description of the program’s capabilities will be given in the chapters that follow. The on-line

help provides further detailed descriptions about each module.

3.1 PROGRAM HIGHLIGHTS

With AutoGrid Pro, the data entry requirements are reduced to a minimum. The input data includes:

Soil resistivities: specify the measured resistivities or the soil layer resistivities and thicknesses

directly (if they are already known).

The grounding grid: use a grid creation wizard, import a preliminary design from a DXF file

created by a CAD package, or else draw the grid directly using the graphical tools of SESCAD or

combine these three methods.

Fault currents: directly specify the component of the fault current injected into the earth by the

grounding grid or let the program compute it based on the network specification. Use the

transmission line databases to quickly describe the network for this calculation.

Safety-related data: specify at what locations earth potentials (and therefore touch and step

voltages) should be computed or let the program decide automatically.

Desired reports and plots: select which reports and plots the program should generate, from an

extensive, predefined list.

Once the data has been entered, simply click Process and let the program do the rest. The program

will compute everything that is necessary, produce the requested reports and plots and display them.

AutoGrid Pro computes only what is needed. Changing the network configuration, for instance, can

affect the fault current component injected into the earth by the grounding grid and therefore the safety-

related quantities. Only these quantities will be recomputed before producing the output. On the other

hand, adding conductors to the grid can affect the grid’s impedance, the fault current component

injected into the earth and the earth potentials, in addition to the safety-related quantities. All of these

quantities are therefore recomputed before producing the output in such a case.


Page 3-2

3.2 USING AUTOGRID PRO

This section briefly describes what can be done with AutoGrid Pro and how to get started with it.

The sections that follow will give more details about the user interface of the program.

3.2.1 Starting AutoGrid Pro

To start the program, simply double-click the AutoGrid Pro icon in your SES Software Program

Group. See Chapter 4 for illustrations of this and the following steps. You will be presented with the

following screen. Users of other software from SES may recognize this screen, which is similar to the

SESCAD program interface. In fact, AutoGrid Pro inherits most of the functionality of SESCAD. Do

not worry if you are not familiar with the SESCAD program, however: this manual does not assume

any prior knowledge of SESCAD.

Figure 3.1 The Main Screen of Autogrid Pro and Some Auxiliary Screens.

The AutoGrid Pro screen also displays a Project toolbox, floating on the right-hand side of the screen,

which is not available in SESCAD. The Project menu item at the top of the screen also gives access

to this new functionality of AutoGrid Pro. (Note: this section will show how to use the Project menu

item to control the application; the same functionality is available most of the time, from the Project

Toolbox.)

The main screen is used to create, modify and view the grounding grid and acts as a controller for the

program. Several other screens are available, coordinated by the AutoGrid Pro Project Toolbox.


Page 3-3

3.2.2 Working with Projects and Scenarios

The design of new grounding systems or the enhancement of existing ones is often an iterative process

in which the design is modified and refined until the goals of the design engineer are attained. In

AutoGrid Pro, these alternative designs are known as Scenarios. A scenario in AutoGrid Pro contains

all of the input data necessary to specify a design, as well as the corresponding computation results,

plots and reports. A Project in AutoGrid Pro is simply a collection of related scenarios.

Before anything can be done with AutoGrid Pro, a project must be created (or an existing one must be

opened).

To create a new project, select Project | New Project. You will be prompted for the location and name

of the project as well as for the name and location of the first scenario of this project. The new project

is created under the filename ‘Project Name’.agp and the scenario under the filename ‘Scenario

Name’.ags where ‘Project Name’ and ‘Scenario Name’ are the names provided for the project and

scenario, respectively. You may drag and drop existing directory to new project and scenario file

location text-boxes. (Note: Experienced CDEGS users may wonder what are the JobID and Working

Directory for the scenario. The answer is that the ‘Scenario Name’ will be used as the JobID and the

selected location for the scenario will be used as working directory. While the concept of JobID and

Working Directory is no longer used in AutoGrid Pro, it may help to know that the database and output

files are still produced using the traditional conventions. For example, the database file for Malt will

be produced in the scenario directory under the name mt_‘Scenario Name’.f21.)

To open an existing project, select Project | Open Project. This will bring up a file browser that

allows you to select an existing project file (with extension “AGP”). You also can drag and drop

existing project directory to Project File Location or File Name text-boxes. Note that a demo project

(called ‘Demo 1’) is available in the folder ‘SES Software\<Version>\Examples\Autogrid

Pro\Demo 1’ (where <Version> is the version number of your SES Software) in your SES Software

Document Folder directory, e.g., C:\Users\Public\Documents. It is also available on your SES

SOFTWARE distribution medium: \Examples\Standard\Autogrid Pro\Demo 1.

Only a single project can be opened at a given time. Therefore, if you attempt to create a new project

or open an existing one while a project is currently open, you will be prompted to save changes to this

last project and the project will be closed before opening the new one.

To save a project, select Project | Save Project or Project | Save Project As. Note that a backup of

the original file is created under the name “Backup of ‘Project Name’”.agp.

Note that you can easily see the contents of the folder containing your project files and of several other

important folders by clicking on Browse to Project Folder on the main toolbar.


Page 3-4

3.2.3 Specifying Data for a Scenario

When a project is open, you always have access to at least one scenario. The data in this scenario can

be edited in the following way.

To specify the soil structure or the soil resistivity measurement data, select Project | Define Soil

Characteristics. This brings up a dialog that allows you to define the structure of the soil (number

of layers, resistivities of the layers, etc…) if it is known or to specify resistivity measurement data

and have the program deduce the soil structure.

To specify the grounding grid and (optionally) the location of the computation points, use the

functionality of the Edit and Tools menus of the main interface. The dialog obtained from

Advanced | Network Energizations and Buried Structures is also useful to specify the fault

current directly (or other forms of grid energization) and to create other structures besides the main

grounding grid.

To specify the circuit and fault current distribution data, use Project | Define Circuit

Characteristics. The data entered in the resulting screens will allow the program to determine

how much current should be injected in the main grounding grid as a result of the fault. You can

use the computed value of the main grounding grid’s resistance or a user specified resistance for

the impedance of the central site of the circuit.

To specify the safety criteria to be used when analyzing the grounding grid, select Project | Define

Safety Criteria. The safety screens allow you to enter the threshold values for safety when

analyzing touch and step voltages as well as some parameters defining the region around the main

grid that should be assessed for safety.

To define which reports and plots the program should produce, use Project | Report Preferences.

The resulting screen offers a wide variety of reports and plots that can be produced whenever the

scenario is processed, including safety reports, touch and step voltage plots, etc…

To control the appearance of the plots, use Project | Graphics Preferences. This allows you to

specify colors, font types and size, etc… that are used when plotting.

To save the scenario, select Project | Save Scenario. Note that a backup of the original file is created

under the name “Backup of ‘Scenario Name’.ags”. To save the scenario under a different filename,

choose Project | Save Scenario As and select an appropriate filename (with the AGS extension). Note

that this will automatically change the scenario name.


Page 3-5

3.2.4 Processing a Scenario

Once the data for a scenario is specified, the grounding safety analysis can begin. To do this, simply

select Project | Process. The program will compute all necessary quantities in the background,

prepare the requested plots and reports and display them. Depending on the input data entered in

the scenario, the processing may include the following steps:

Saving of the scenario’s data

Computation of an appropriate soil structure from the measured soil resistivities

Determination of an appropriate safety zone where the analysis should be conducted

Computation of the main grid resistance

Determination of the distribution of the fault current throughout the network

Computation of the earth potentials and grid GPR at the fault site

Computation of the safety limits for touch and step voltages

Computation of touch and step voltages, and safety analysis

Production of reports and plots

Other optional steps may include ampacity assessment, etc…

When the processing begins, a window appears and displays messages regarding the progress of the

computations.

3.2.5 Adding New Scenarios

Once the processing is complete, the results can be reviewed to determine if the design is satisfactory.

If not, the design can be modified and the above steps repeated. There are two ways to modify the

design: you can modify the existing scenario directly, in which case the original data is lost, or you

can create a new scenario and modify that one.

To create a new scenario, select Project | New Scenario. You will be prompted to provide a name for

the new scenario as well as (optionally) the name of an existing scenario to be used as a reference.

When a valid reference scenario is provided, the program creates a copy of that scenario under the

new name. This is convenient when you want to examine small design variations from one scenario

to the next.

You can also open one of the project’s existing scenarios by choosing Project | Open Scenario. You

will be presented with a list of the scenarios that are presently in the project, from which you can select

the desired one.


Page 3-6

3.2.6 Closing a Project

To close a project, select Project | Close Project. This will close the current project, but not the

program itself. Closing the window containing the drawing of the main grid is also interpreted by

AutoGrid Pro as a signal to close the project.

When the project is closed, you can still use AutoGrid Pro much as you would SESCAD, i.e., the

project functionality is disabled but everything else is available. Use Project | Open Project to open

another project.

The most recently used projects are listed at the end of the Project menu and in the Open Project

dialog, for quick access.

3.2.7 Ending Your AutoGrid Pro Session

To quit the application and terminate the AutoGrid Pro session, use File | Exit. The program will

optionally prompt for those files that need saving before terminating.

Chapter 4. Creating a Project and Scenario

Page 4-1


CREATIONG A PROJECT AND SCENARIO

In this chapter, we will describe in detail how to get started by creating a new project which contains

a first scenario.

4.1 START-UP PROCEDURES


Page 4-2

In the SES Software <Version#> group folder, where <Version#> is the version number of the

software, you should see the icons representing Autogrid Pro, AutoGroundDesign, CDEGS, Right-

of-Way, SESEnviroPlus, SESShield-3D and SESTLC software packages, as well as four folders.

The Documentation folder contains help documents for various utilities and software packages. The

Program Folders provides shortcuts to programs, installation and projects folders. The System folder

allows you to conveniently set up security keys. Various utilities can be found in the Tools folder. The

main function of each software package and utility is described hereafter.

SOFTWARE PACKAGES

Autogrid Pro provides a simple, integrated environment for carrying out detailed grounding

studies. This package combines the computational powers of the computation modules RESAP,

MALT and FCDIST with a simple, largely automated interface.

AutoGroundDesign offers powerful and intelligent functions that help electrical engineers design

safe grounding installations quickly and efficiently. The time devoted to design a safe and also

cost-effective grounding grid is minimized by the use of automation techniques and appropriate

databases. This module can help reduce considerably the time needed to complete a grounding

design.

Right-of-Way is a powerful integrated software package for the analysis of electromagnetic

interference between electric power lines and adjacent installations such as pipelines and

communication lines. It is especially designed to simplify and to automate the modeling of

complex right-of-way configurations. The Right-of-Way interface runs the TRALIN and SPLITS

computation modules and several other related components in the background.

SESEnviroPlus is a sophisticated program that evaluates the environmental impact (radio

interference, audible-noise, corona losses, and electromagnetic fields) of AC, DC or mixed

transmission line systems.

SESShield-3D is a powerful graphical program for the design and analysis of protective measures

against lightning for substations and electrical networks. Its 3D graphical environment can be used

to model accurately systems with complex geometries.

SESTLC is a simplified analysis tool useful to quickly estimate the inductive and conductive

electromagnetic interference levels on metallic utility paths such as pipelines and railways located

close to electric lines (and not necessary parallel to them), as well as the magnetic and electric

fields of arbitrary configurations of parallel transmission and distribution lines. It can also compute

line parameters.

CorrCAD tackles a large variety of cathodic protection design tasks and related issues, onshore

and offshore, and can also predict the degree of corrosion control provided by a system. A typical

application for corrosion control includes Impressed Cathodic Current Protection systems (ICCP)

and use of sacrificial anodes in anodic protection systems, where anodic current is impressed on

corroding material to enforce passivation. Another application is to estimate the effect of stray

currents such as those produced by HVDC electrodes or dc rail traction systems on the corrosion

of buried metallic structures. CorrCAD can evaluate the corrosion status of the structure and help


Page 4-3

optimize the location and characteristics of the corrosion protective system (such as ICCP) to

minimize stray current interference effects on protected structures such as pipelines.

CDEGS is a powerful set of integrated software tools designed to accurately analyze problems

involving grounding, electromagnetic fields, electromagnetic interference including AC/DC

interference mitigation studies and various aspects of cathodic protection and anode bed analysis

with a global perspective, starting literally from the ground up. It consists of eight computation

modules: RESAP, MALT, MALZ, SPLITS, TRALIN, HIFREQ, FCDIST and FFTSES. This is

the primary interface used to enter data, run computations, and examine results for all software

packages other than Right-of-Way, Autogrid Pro, AutoGroundDesign, SESTLC, SESShield-3D

and SESEnviroPlus. This interface also provides access to the utilities listed below.

CDEGS is accessible via a modern, user-friendly and flexible main interface. A legacy interface,

called CDEGS-Legacy, is also available.

TOOLS

AutoTransient automates the process required to carry out a transient analysis with the HIFREQ

and FFTSES modules

CETU simplifies the transfer of Right-of-Way and SPLITS output data to MALZ or HIFREQ. A

typical application is the calculation of conductive interference levels in an AC interference study.

F05TextEditor is an enhanced text editor that recognizes the command structure of the module

indicated by the file prefix. The program provides syntax highlighting and a command parameter

identification tooltip to greatly simplify manual editing of an .f05 file.

FFT21Data extracts data directly from FFTSES’ output database files (file 21) in a spreadsheet-

compatible format or in a format recognized by the SESPLOT utility.

GraRep is a program that displays and prints graphics or text files. For more information on

GraRep see Chapter 6 of the Utilities Manual or invoke the Windows Help item from the menu

bar.

GRServer is an advanced output processor which displays, plots, prints, and modifies

configuration and computation results obtained during previous and current CDEGS sessions.

GRSplits plots the circuit models entered in SPLITS or FCDIST input files. This program greatly

simplifies the task of manipulating, visualizing and checking the components of a SPLITS or

FCDIST circuit.

GRSplits-3D is a powerful interactive 3D graphical environment that allows you to view and edit

the circuit data contained in SPLITS input files and to simultaneously visualize the computation

results.

RowCAD is a graphical user interface for the visualization and specification of the geometrical

data of Right-of-Way projects. Its 3D graphical environment can be used to visualize, specify and

edit the path data of Right-of-Way, and to define the electrical properties of those paths.


Page 4-4

SESAmpacity computes the ampacity, the temperature rise or the minimum size of a bare buried

conductor during a fault. It also computes the temperature of bare overhead conductors for a given

current or the current corresponding to a given temperature, accounting for environmental

conditions.

SESBat is a utility that allows you to submit several CDEGS computation module runs at once.

The programs can be run with different JobIDs and from different Working Directories.

SESCAD is a CAD program which allows you to create, modify, and view complex grounding

networks and aboveground metallic structures, in these dimensions. It is a graphical utility for the

development of conductor networks in MALT, MALZ and HIFREQ.

SESConductorDatabase gives access to the SES Conductor Database. It allows you to view the

electrical properties of conductors in the database, and to add new conductors to the database or

modify their properties.

SESConverter is a DXF-DWG Converter tool that can be used to import CAD based files to

various SES software package compatible input files or export various SES software package input

command files to CAD files compatible with the DXF or DWG format. The program allows

filtering of data to be imported aided by a 2D viewer of selected data, to avoid excessive conductor

creation in the SES software package compatible files.

SESCrossSection provides an interactive interface with direct visual system representation for the

specification of conductor characteristics and locations within a conductor path cross-section. The

program allows data specification for eventual use in CorrCAD, Right-of-Way, Cable and

Conductor modes of SESLibrary, SESeBundle, and Circuit, Group and Single modes of the

TRALIN module.

SESCurveFit is a general curve fitting tool with a special focus on "Polarization curves" used in

CorrCAD. It incorporates a curve digitizer utility as well.

SESeBundle finds the characteristics of an equivalent single conductor accurately representing a

bundle of conductors, as far as their series impedance is concerned. This utility is particularly

useful to simplify models in modules, such as HIFREQ, where reducing the number of conductors

is important to keep the computational time low.

SESEnviroPlot is an intuitive Windows application that dynamically displays computation data

produced by the SESEnviroPlus software module.

SESFcdist is an interactive and flexible interface to prepare and run input files, and view results

from, the FCDIST computation module.

SESFFT is a Fast Fourier Transform computation module designed to help you automate time

domain (lightning and switching surges) analyses based on frequency domain results obtained

from CDEGS computation modules such as SPLITS, MALZ, and HIFREQ. The forward and

inverse Fast Fourier transformations, the sample selection of the frequency spectrum, and related

reporting and plotting functions have been automated in SESFFT.


Page 4-5

SESGSE rapidly computes the ground resistances of simple grounding systems, such as ground

rods, horizontal wires, plates, rings, etc., in uniform soils. SESGSE also estimates the required size

of such grounding systems to achieve a given ground resistance.

SESImpedance computes the internal impedance per unit length of long conductors of arbitrary

geometry and composition, and whose cross-section does not vary over the length of the conductor.

The program uses the Finite Element Method (FEM) for calculating the electrical characteristics

of conductors and is capable of handling conductors of arbitrary shapes and realistic material

properties. The calculations fully account for skin effect, and can be carried out at low or high

frequency.

SESLibrary allows you to inspect the properties of a large number of components that can be part

of models for many SES Software computation modules. It currently includes a comprehensive

database of conductors as well as several power cables.

SESPlot provides simple plots from data read from a text file.

SESPlotViewer is a tool used by SESEnviroPlus for plot rendering.

SESResap is an interactive and flexible interface to prepare and run input files and view results

from the RESAP computation module.

SESResultsViewer processes the computation data and results of all computation modules in

CDEGS, offering a complete solution for displaying the plots and reports in an integrated viewer.

It presents a light layout with intuitive organization of its settings that use sensible defaults that, in

turn, allow for a fast configuration of the settings in order to achieve the desired output results.

SESScript is a script interpreter that adds programming capabilities to SES input files. SESScript

can systematically generate hundreds of files from a single input file containing a mixture of the

SICL command language and scripting code and user-defined parameter ranges and increments.

SESShield provides optimum solutions for the protection of transmission lines and substations

against direct lightning strikes and optimizes the location and configuration of shield wires and

masts in order to prevent the exposure of energized conductors, busses and equipment. It can also

perform risk assessment calculations associated with lightning strikes on various structures.

SESSystemViewer is a powerful 3D graphics rendition software that allows you to visualize the

complete system including the entire network and surrounding soil structure. Furthermore,

computation results are displayed right on the system components.

SESThreshold is an application for computing threshold limits, as recommended by industry

standards, for touch and step voltages. It is coupled with the Zone Editor application, allowing

zones where different threshold limits are applicable to be defined.

SESTralin is an interactive and flexible interface to prepare and run input files, and view results

from, the TRALIN computation module.


Page 4-6

SoilModelEditor is a standalone module with an interactive graphical interface that assists

in the creation of soils models for all relevant target SES modules.

SoilModelManager is a software tool that automates the selection of soil model structures that

apply during various seasons.

SoilTransfer utility allows you to transfer the soil model found in several SES files into several

MALT, MALZ or HIFREQ input (F05) files.

TransposIT is a tool for the analysis of line transpositions on coupled electric power line circuits.

To ensure that voltage unbalance is kept within predefined limits, it allows the user to determine

the optimal number of power line transpositions and their required locations.

WMFPrint displays and prints WMF files (Windows Metafiles) generated by CDEGS or any

other software.

During this tutorial, for simplicity, we will be using the Autogrid.Pro icon to carry out most of the

input and output tasks. We will refer to the other utility modules when appropriate.

4.1.1 Creating a New Project

Click on the New tab to request a new

project workspace. You will first see the

following screen in which a Project

Name Project1 and a Scenario Name

Scenario1 is automatically assigned by

the program.

In this tutorial, we will create a project

called AGP Tutorial under the folder

D:\Projects\AGP Tutorial (you can use

any existing folder on your PC, or create

a new one). We will first change the default Project File Location to D:\Projects, and then we enter

AGP Tutorial in the Project Name field. A sub-folder called AGP Tutorial under D:\Projects is

automatically offered while you are typing the project name AGP Tutorial. You can manually rename

the project folder name (under Project File Location) if you wish this name to be different from the

Project Name. However, to keep things simple, it is usually recommended to keep the same name for

the Project File Location and the Project Name.


Page 4-7

To create a scenario called Initial Design,

enter Initial Design under Scenario Name.

Again, a sub-folder called Initial Design

under D:\Projects\AGP Tutorial is

automatically offered while you are typing

the scenario name Initial Design. It is also

recommended to keep the same name for the

Scenario File Location and the Scenario

Name.

Click the Create button. The AGP Tutorial

project and the Initial Design scenario are

created and ready for use. You can now

proceed to Section 4.1.3.


Page 4-8

4.1.2 Opening an Existing Project

Click on the Existing tab to browse for an existing project file. Navigate to the AGP Tutorial folder

under your Project folder, then double-click the file AGP Tutorial.agp. This will load the project.

4.1.3 Using the Project

The buttons on the Project Toolbox are now active for you to enter data. The AutoGrid Pro project

toolbox acts as a quick launch pad for the other data entry screens of the application.

Project: allows you to open new or existing projects and scenarios: analogous to the File | Open

and File | New commands in Microsoft® Applications.

Reports: Allows you to select which reports and plots you wish to generate.

Setup: Customizes the appearance of plots.

Wizard: Loads the AutoGrid Pro Wizard that guides you through a typical session (not yet

available).

Settings: Specify your general personal preferences here.

Soil: Enter soil description or measurements.

Grid: Enter grid data that you have not specified graphically.

Circuit: Specify the power lines connected to the substation if you wish to have the program

calculate the split of fault current between the grounding grid and earth return conductors such as


Page 4-9

static wires and neutral conductors. Note that conductive earth return wires can decrease fault

current injected into the earth by the grid by 50% or more.

Safety: Specify the criteria to be used in the safety analysis.

GRServer: Start the GRServer program (an advanced graphics processor program) to display

graphically the results of a scenario in greater detail.

Process: Initiate the computations, which end with the production of all requested plots and

reports.

4.1.4 Files that Are Part of the Project

When you create a new project, several files and folders are automatically created on your hard disk.

The folders and files created in the previous sections can be viewed in the following Windows

Explorer screen.

The file AGP Tutorial.agp is a project file for the AGP Tutorial project. It contains the information

regarding all the scenarios defined under this project. Under the scenario subfolder Initial Design, you

will find a file Initial Design.ags, and two other files MT_Initial Design.F05 and FC_Initial

Design.F05. These files were created the moment the scenario Initial Design was created. The file

Initial Design.ags stores the data for the Scenario Initial Design. For those who have used CDEGS

software before, you may recognize that the two files MT_Initial Design.F05 and FC_Initial

Design.F05 are the input files for the MALT and FCDIST programs, respectively.

With a project and scenario defined, we are now ready to enter soil resistivity data (Chapter 5), define

the initial design of the grounding grid (Chapter 6), prepare the fault current data (Chapter 7), evaluate

the performance of the initial grid design (Chapter 8), and make final refinements to the design.

Chapter 5. Soil Resistivity Data Entry

Page 5-1


SOIL RESISTIVITY DATA ENTRY

5.1 A HORIZONTAL TWO-LAYER SOIL MODEL

The data values listed in Table 2-1 at the end of Chapter 2 were entered as input to the soil resistivity

analysis module of the AutoGrid Pro package. This consists of the following information:

Spacing Between Probes: The distance between adjacent measurement probes.

Apparent Resistance (V/I): The apparent resistance measured at each probe spacing.

Current Probe Depth: The depth to which the current injection electrodes were driven

into the earth. This value influences the interpretation of soil resistivities at short electrode

spacings. It is an optional field data.

Potential Probe Depth: The depth to which the potential probes were driven into the earth.

This value also influences the interpretation of soil resistivities at short electrode spacings.

It is an optional field data.

The soil resistivity interpretation module RESAP is used to determine equivalent horizontally layered

soils based on the site measurements. Although RESAP is capable of producing multi-layered soil

models, it is preferable to try to fit the measured results to the simplest soil structure (i.e., a two-layer

model), at least initially. This minimizes the time required for the computations. When a two-layer

soil model is selected, the computation results lead to an equivalent two-layer soil structure such as

the one shown in Table 5-1. The “RMS Error” computed by RESAP (see Section 8.3.3) provides a

quantitative indication of the agreement between the measurements and the proposed soil model. The

grounding system resistance computed by the grounding module MALT (see Sections Chapter 6 and

8.3.4) is also shown. Note that the resistance shown here was computed for the initial design of the

grounding system of the East Central Substation.

Layer Resistivity

(-m)

Thickness

(Meters)

Top

Bottom

297.08

65.85

0.67

Table 5-1 Two-Layer Soil Model Computed Using Data from Table 2-1

RMS Error: 16.95%

Grid Impedance: 0.538

The following section describes the steps required to determine the soil model shown in Table 5-1.


Page 5-2

5.2 SOIL RESISTIVITY DATA ENTRY

Click the Soil button located in the

Project Toolbar to define the soil model.

The Soil Structure screen will appear,

without data and you are now ready to

enter it.

The soil model can be defined by

specifying measured resistivity data

(default setting shown above), in which

case the program will compute an

appropriate soil structure, or by

specifying the soil structure explicitly.

When the soil structure is specified

explicitly by selecting the Use Specified

Soil Structure Characteristics option at

the very top of the screen, the following

data entry screen is shown; use it to enter

details of the soil model that is desired.

The earth structures that can be analyzed include:

Uniform soil model (default)

Horizontally layered soil (any

number of layers)

Vertically layered soil (any

number of layers)

Hemispherical soil model: three

regions delimited by two concentric

hemispherical boundaries

Cylindrical soil model: two

regions delimited by a vertical or

horizontal cylindrical boundary

Hemispheroidal soil model: two

regions delimited by a

hemispheroidal boundary

Inclined soil model: two regions

delimited by an inclined layer

Arbitrary heterogeneity soil

model: any number of regions of any shape formed by 6 surfaces and 8 vertices

In this tutorial, the soil model will be deduced based on the soil resistivity measurements presented in

Section 2.4.


Page 5-3

In the following, it will be assumed that the reader is entering the data as indicated in the instructions.

Note that it is advisable to save your work regularly by selecting Project | Save Project, or by pressing

the Ctrl + S key combination as a shortcut. The data, entered up to that point, will be saved in the

RESAP input file RS_Initial Design.F05 in the “Initial Design” folder, in addition to the project and

scenario files.

If you intend to enter the data manually, proceed with this section; otherwise, you can import all the

data by proceeding as follows:

Importing Data

In the Soil Structure screen, click the Import button. Select the file “\Files For Import\RS_Initial

Design.F05” in the dialog, and then click OK. The data described in the next section will be loaded

and you will not have to enter it yourself.

In the Soil Structure screen, enter

the measured apparent resistances at

the substation site (see Table 2-1).

This screen also allows you to

specify the Measurement Method

used to gather the data (the default

setting is Wenner), the Type of data

recorded, and the field measurement

data obtained. Click the radio

buttons General, Wenner and

Schlumberger to understand the

differences between them. You can

immediately plot the raw data in a

linear/linear, log/linear or log/log

fashion to examine the shape of the

curve and spot irregularities in the

measurements.

Click the Properties button to bring

up the screen shown in the following

page. This screen allows you to provide comments under Case Description and to select the System

of Units. You can provide comments that apply to the entire project (the Project Level comments), to

the current scenario (the Scenario Level comments), or to any individual module. In this case, we

enter a description of the soil resistivity analysis under Measured Soil. These comments are echoed

in the output file of the RESAP program.

You can enter comments for the other modules of the software, if you wish to, by clicking on the

buttons of the top of the screen; we will do this later, in other parts of the tutorial. (Here, enter “A

simple substation grounding grid analysis using AutoGrid Pro.” in the Project Level comment and

“Initial Design: a linearly spaced grid, without rods.” in the Scenario Level comment.)


Page 5-4

A Run-ID Initial Design is automatically entered in the Run-Identification data entry field. The Run-

ID is useful in identifying all the plots which will be made later in Section 8.3.3.

In this tutorial, the Metric System of Units is used. The

system of units selected here applies to the entire

scenario, and all the computation modules. When

changing the units, you have the option to convert the

data to the new system of units or to leave the data as is.

By default, the data is left as is. To convert the data, click

on “Data Conversion Options”, and follow the

instructions in the resulting screen.

Focusing on any field in this screen (e.g., by clicking on

the field or by clicking on a screen button, without

releasing the mouse button, then dragging the mouse off

the screen button, then releasing the mouse button), then

pressing the F1 key will bring a help text related to the

focused field, or to the screen as a whole. This is true of

all screens in AutoGrid Pro.

Click the OK button to return to the Soil Structure main

screen. By default, the RESAP program will provide a

soil model which is the best fit to your data.

If you wish to see how sensitive the computed resistivity curve is to changes in soil layer thicknesses

or resistivities then click on the Advanced button and suggest your own soil model. Select the User-

Defined option in the resulting screen if you wish to specify the number of layers and, optionally, the

characteristics of selected layers. In this tutorial, we select the two-layer soil model.

You can actually Lock or Unlock the resistivity and the

depth of each layer. Note that when the soil characteristics of

one layer (resistivity or depth or both) are locked, these

values will not be altered during the least-square iterative

minimization process. By leaving the Locking Options

column blank (or setting it to Unlock), you are leaving the

program free to determine suitable values for the associated

soil layer characteristics. If you prefer to specify your own

values, you should use the Lock option, then select which

item(s) to lock under Lock/Unlock Item.

In this tutorial, we will select a User-Defined soil with 2

layers, and let the program determine the properties of the

layers automatically. Click the OK button to return to the Soil

Structure main screen, then click OK in that screen to

complete the data entry for the soil structure specification.

Chapter 6. Initial Grounding Grid Design

Page 6-1


INITIAL GROUNDING GRID DESIGN

In this chapter, we will show how to create a detailed computer model of a grounding system.

The determination of the grounding grid performance is carried out by the MALT computation

program, which computes the grounding grid resistance, ground potential rise, earth potentials, and

thereby touch and step voltages. In fact, you can model several distinct grounding systems at the same

time, each energized with a different current or voltage: MALT will determine how they all influence

one another, allowing you to determine transferred potentials, touch voltages and step voltages at any

location. Each grounding grid (or “electrode”) consists of a group of cylindrical conductors with any

orientations and positions, although they must all be buried. All conductors you identify as belonging

to given grounding grid are automatically interconnected for you (by means of invisible cables) and

all conductors are assumed to have negligible longitudinal impedance - a fair assumption for typical

substation. As a rule of thumb, a computed ground resistance less than 0.5 indicates a possible need

for modeling by other software which does account for conductor impedance, such as SES’ MALZ

software module.

At least one electrode (called “MAIN”) must be defined. It is normally used to model the main

grounding grid of the system under study (most studies only do examine a single grid). Other

electrodes (called “RETURN Ground” and “BURIED Structures”) can be defined; although they can

be used in many different ways, they are typically used to model a return electrode that collects all the

current injected in the main, (e.g., when simulating a ground impedance field test) and passive buried

structures (such as pipes or floating fences not connected to the main substation grid) which are within

the zone of influence of the main grounding grid.

In this tutorial, we will only need to define a MAIN electrode. It will be used to model the main

grounding grid at East Central Substation. For the initial design, a 100 m by 60 m grid will be studied.

6.1 DATA ENTRY

As for the soil resistivity data entry, you can enter the data manually by following the steps described

in this section; or if you do not wish to do so, you can import all the data required for this tutorial by

proceeding as follows:

Importing Data

Select Project | Import File and select the file “\Files For Import\MT_Initial Design.F05”. The data

described in the next section will be loaded and you will not have to enter it.


Page 6-2

While most of the data entry regarding

the grid will be carried out using the

graphical tools available from the main

screen of AutoGrid Pro, some extra data

can be specified in the screen below.

For those who have used SESCAD

before, you have probably already

noticed that the graphical tools are

simply the SESCAD program of the

CDEGS package. Chapter 10 of

Utilities Manual is devoted to

describing in detail how to use

SESCAD. This manual is available in

the PDF folder on your SES

SOFTWARE distribution medium

under the filename Utilities.PDF. You

are strongly encouraged to read Chapter

10 of this manual, in order to learn how

to unleash the full power of this

graphical interface.

We will begin by defining some properties of the main grid. Select Project | Define Grounding

System Energization and Buried Structures to load the Grounding System Energization screen.

The data to be provided on this screen consists mainly of the current or voltage magnitude to be

impressed upon each grounding system modeled. It is also used to specify the presence of buried

metallic structures other than the MAIN grounding grid. Note that there is no fundamental difference

between the MAIN grounding grid, the RETURN ground, and any Buried Structures you may wish to

model. The only real differences are that the MAIN grounding grid must be defined, whereas the other

buried system are optional; furthermore this MAIN grounding grid must be energized by a non-zero

voltage or current, whereas the other buried systems, if they exist, may be either energized or left

floating.

By default, the option Use Value Calculated by the Fault Current Distribution Module (if

available) is selected. This instructs the program to use the fault current calculated in the Fault Current

Analysis module as the energization current for the grid. If the fault current distribution calculation is

not required, you can enter the magnitude of the fault current component to be injected into the earth

by the grounding grid: do this under the option Use Specified Value. The value specified in the

Magnitude field will also be used when the calculated value of the fault current is not available yet

(usually because the data specification for the fault current distribution module is incomplete).


Page 6-3

Click the Properties button to bring up the screen shown

below. This screen allows you to provide comments

under Case Description and to select the System of

Units, as was done in the Soil Resistivity Data Entry in

Section 0.

The comments are echoed in the output file of the MALT

program. Again, a Run-ID Initial Design is

automatically entered in the Run-Identification field

and the Metric System of Units is selected. Click the

OK button, then the OK button to return to the Auto

Grid Pro main screen.

Next, we complete our description of the system under

study with the graphical tools of AutoGrid Pro: these

will allow us to rapidly describe the grounding grid and

the points at which earth potentials (and therefore touch

and step voltages) are to be computed. In our initial

design shown in Figure 2.2, we require a 100 m x 60 m

(328 feet x 197 feet) rectangular grid, buried at a depth

of 0.5 m (1.64 feet), and made of 9 linearly spaced

conductors parallel to the y-axis and 7 linearly spaced conductors parallel to the x-axis. Each conductor

has a radius of 0.6 cm (0.24 inches) and will be subdivided into 10 sub conductors or “segments” for

improved accuracy of the results (see below for further details on conductor segmentation). The origin

of the coordinate system used to specify the grounding grid is chosen to be at the bottom left-hand

corner of the grid.

To enter this data, select Create Object from the Edit menu. The Create Object dialog allows you

to define Conductors as well as observation Profiles (or observation points; more about those later).

Select the Detailed Grid option under Conductors and enter the coordinates of three corners of the

grid: a, b and c (as identified in the figure below). Nab indicates the total number of conductors parallel

to the x-axis, and Nac indicates the total number of conductors parallel to the y-axis. Note that you

should specify the Z coordinate as positive, to indicate that they are buried. This is generally true in

AutoGrid Pro, which considers the positive Z direction to be going down into the earth.


Page 6-4

Click the Characteristics button and assign a radius of 0.006 m (0.019 ft) to all conductors. The value

in Subdivision # specifies a minimum segmentation number for all the conductors. In this tutorial, we

leave the value of this field at 1 since the conductor segmentation generated by the node subdivision

feature is already adequate. This is explained in greater detail in the “Conductor Subdivision” inset

(see below).

The grid is now created. Click on Apply to transfer the grid to the main drawing window, then click

on Close to close the Create Object window. Note that, for simplicity, we have defined a grid with

uniformly spaced conductors in this example. However, in many cases, grids with uniformly spaced

conductors are not as efficient as grids with conductors more closely spaced towards the edge of the

grid than at its center as will be demonstrated later in this tutorial.


Page 6-5

To examine the touch and step voltages in and around the substation, the earth potentials should be

computed at observation points covering an area extending about 3 meters outside the substation. As

will be shown in the next section, you can let the program determine the location of suitable

observation points. On the other hand, for finer control you can also specify the observation points

explicitly. This is what we will do here.

We will define a profile containing evenly spaced points and replicate this profile using the surface

entry fields. This will produce a grid of observation points at the surface of the earth above the

grounding system. Note that if you have already imported the data from a file, the computation profiles

are already specified in the Auto Grid Pro screen. Avoid generating a duplicate set of profiles.

As for the grounding grid, the observation points can be defined using the Create Object command

under the Edit menu. Select the Detailed Surface option in the Create Object screen and enter the

data as shown in the screen below, then click on Apply and Close. This defines a rectangular surface

of observation points centered above the grid. The points are evenly spaced, being 1 meter apart both

along the x and y axes.

Conductor Subdivision

As with many computer models of physical systems, the theory behind the MALT program

requires a discrete representation of a continuous phenomenon, namely the distribution of

the current discharged to earth by the grid’s conductors. The assumption made by the

program is that every conductor segment discharges current uniformly along its length. In

order for this to represent reality accurately, the conductor segments must be small enough.

There are several ways to generate conductor segments from the specified grid conductors.

First, the program automatically breaks all conductors at every conductor intersection. This

is called the node subdivision process. Usually, this is already enough to guarantee accurate

results, so that further intervention is unnecessary. A second, simple way to generate the

conductor segments is to enter the total number of segments to be generated by the program

in the Desired Number of Segments field of the Grounding System Energization

screen. If the total number of segments obtained after the node subdivision is smaller than

the desired number of segments, the program will break the longer conductors into two

equal length pieces, until the desired number of segments is reached. Another way is to

proceed as above, by specifying explicitly the number of segments desired for each

conductor individually. This method gives a very fine control over the segmentation

process.

Note also that it is often a good idea to do two sets of computations, the second one using

a larger number of segments (but otherwise identical to the first). The results should not

change by more than a few percent. This verifies explicitly if the number of segments used

in the computations is adequate.


Page 6-6

The data is specified by defining an observation profile, that is a linear group of NPoints evenly spaced

observation points, then by replicating this profile NProfiles times along the direction defined in

Profile Step. The original profile starts at (-3, -3, 0) and the profile points are separated by 1 meter

along the X axis.

Note that the values of NPoints and NProfiles always include the starting point and profile,

respectively. With a total of 107 points per profile and 67 profiles, the observation surface extends

from x = -3 m to x = + 103 m and from y = -3 m to y = 63 m, and therefore extends past the perimeter

of the grid by 3 meters.

Once the profiles are created, they are superposed on top of the grid already defined, which provides

a convenient way to check the positions of the observation points with respect to the grid. You can

also turn off the display of the observation points by unchecking the Profiles options in the View

menu.


Page 6-7

At this point, you have completed the data entry for the grid specification.

Chapter 7. Fault Current Distribution Analysis

Page 7-1


FAULT CURRENT DISTRIBUTION ANALYSIS

7.1 INTRODUCTION

The touch and step voltages associated with the grounding network are directly proportional to the

magnitude of the fault current component discharged into the soil by the grounding network1. It is

therefore important to determine how much of the fault current returns to remote sources or external

grounding via the shield wires and neutral wires of the transmission lines and distribution lines

connected to the substation under study, in this case, East Central Substation. In other words, the

current discharged into the East Central Substation grounding system is smaller than the maximum

available fault current, because a portion of the fault current returns via the shield wires and neutral

wires of the power lines connected to the East Central Substation and local transformer contributions

are disregarded. In order to be able to determine the actual fault current split, a model of the overhead

transmission line network (and, when present, distribution neutrals and associated grounding) must be

built. Before this, however, it is necessary to calculate transmission and distribution line parameters

such as self and mutual inductive impedances, at representative locations.

This work is described in the present chapter. In this study, we assume a single-phase-to-ground fault.

The computation module FCDIST is used to compute the fault current distribution. For more

complicated fault scenarios, the computation modules TRALIN and SPLITS of the CDEGS package

can be combined to complete the task: in this case, the line parameters are computed using the TRALIN

module, then the resulting parameters are used by the SPLITS module to compute the fault current

distribution. The How To… Engineering Guide entitled “Analysis of AC Interference Between

Transmission Lines and Pipelines” gives a detailed example on how to use TRALIN and SPLITS to

compute the fault current distribution.

As mentioned in Chapter 2, we assume that the highest fault current discharged by the East Central

grounding grid occurs for a 230 kV single-phase-to-ground fault at the East Central Substation on

Phase B2 of Circuit 2. Note that if autotransformers are involved it becomes particularly important to

examine the currents flowing into the substation in all phases of all circuits (at all voltage levels) for

the 230 kV fault in order to correctly assess the situation.

1 Strictly speaking, circulating currents flowing in grounding grid conductors from the fault location to local transformer

ground connections and to static and neutral wire ground connections also contribute to touch voltages, particularly in

large grounding grid in low resistivity soils. For typical substation applications, however, this component is relatively

small.


Page 7-2

7.2 PREPARATION OF THE INPUT DATA

The fault current distribution is computed using the FCDIST (Fault Current Distribution) computation

module. The goal of this analysis is to find the fraction of the total fault current that is discharged in

the grounding grid under study. The important data for such an analysis consists of:

The impedance of the grounding grid under study. In the program, this grounding grid is

referred to as the Central Site.

The fault current sources: these are called Terminals in the program. The data to be specified

includes the magnitude and phase angle of the contribution of each terminal to the fault current,

as well as the impedance of the grounding grid at each terminal.

The electrical characteristics of the transmission lines connecting the Terminals to the Central

Site. This normally includes the geometrical configuration of the faulted phase conductors and

of the shield wires as well as the type of shield wires used; alternatively, the line impedances

can be specified explicitly. In addition, representative ground resistances of the transmission,

and distribution line towers and poles must be specified, in order to take credit for the full

benefit provided by these.

The model allows only a single-phase wire per power line; therefore, only the faulted phase and the

neutral conductors (or shield wires) are represented; the other phases are ignored. You may, however,

include an approximate of the contributions of the other phases by specifying the vector sum of the

currents flowing in the three phases of the circuit of interest as the current flowing in the faulted phase.

The average height and lateral position of the conductor bundle associated with the faulted phase are

specified in terms of their Cartesian coordinates. The positions of up to two static or neutral wires per

power line are specified in a similar manner. A concentric neutral can be modeled instead of simple

static or neutral conductors: this shield is modeled as a bundle of small conductors arranged to form a

cylinder resembling the concentric neutral.

The following input data can be extracted from the description of the circuit in Section 2.2.

Central Site

Name: East Central

Ground Impedance: To be supplied automatically by the grounding module each time the grounding

system is modified and upon its initial creation.

Terminals

Static Wires: 7 No. 8 Alumoweld


Page 7-3

Terminal (Source Substation) Characteristics Transmission Line Characteristics

Name Fault Current

Contribution

(Amps)

Ground

Impedance ()

Span

Length (m) Number of

Spans

Tower Ground

Resistance ()

Greenbay 1226 – j 5013 0.2 330 64 10

Hudson 722 – j 6453 0.3 330 25 28

Newhaven 745 – j 5679 0.3 330 33 28

Table 7-1 Terminal Information for the Example Study: Single-Phase-to-Ground Fault

at East Central Substation

In this study, the static wires are located symmetrically with respect to the center line of the tower, at

a height of 35 m (115 feet) and at a distance of 7.3 m from the center of a tower (see Figure 2.3). You

will note that the primary purpose of the fault current analysis is to determine how much of the fault

current flows into the grounding system of the substation under study (i.e., the Central Station) during

a fault at that location and how much does not, because of alternate ground return paths provided by

static and neutral wires. The analysis will also determine the magnitude of the current that returns to

each power source, through the earth, via the terminal grounds and determine the influence of the

mutual impedances between the phase and static/neutral wires. This latter effect manifests itself as a

“trapped” current in the static/neutral wires. The computation results provide the self-impedances of

the static/neutral wires as well as the mutual impedances between the phase and static/neutral wires

for each power line modeled.

In this chapter, we will show how to set up the computer model of the transmission system connected

to East Central Substation

7.2.1 Data Entry

On the Project toolbar, click on the Circuit button. This will bring the Network Fault Current

Distribution window. As for the other parts of this tutorial, you can manually enter the data associated

with this tutorial by following the steps described in this section; or, if you do not wish to do so, you

can import all the data by proceeding as follows.

Importing Data

In the Network Fault Current Distribution screen, click on the Import button. Select the File

Name “\Files For Import\FC_Initial Design.F05”, then click OK. The data described in the

remainder of this chapter will be loaded and you will not have to enter it.


Page 7-4

The Network Fault Current

Distribution screen contains two tabs

(Central Site and Terminals) dedicated,

as their names indicate, to the data entry

for the Central Site and Terminals,

respectively. We will begin by entering

the Central Site data.

After specifying the name of the Central

Site (“East Central”), the impedance of the

grounding grid must be provided. At this

point, this impedance is unknown, since it

depends on the actual design of the

grounding grid. In fact, as the design of the

grounding grid is refined in the course of

the study, the value of this impedance will

change. The simplest way to specify the

impedance is to let the program compute it

from the data defining the grid: this option

can be selected by choosing Deduce from

Grounding Computations.

This screen also allows you to specify the

computation frequency (typically 60 or 50

Hz) and the average electrical

characteristics of the soil in the region

covered by the electrical network. These

properties are used when computing the

self and mutual impedances of the

transmission lines connecting the

terminals to the central site. These are not

highly sensitive to the soil resistivity, so an

order of magnitude estimate of the average

soil resistivity usually suffices. The soil’s

relative permeability is usually equal to

1.0.

The Terminals tab (shown next) is used to

define the properties of all terminals. We

will show how to specify the data for one

terminal completely, and then show some

shortcuts to rapidly create the other

terminals.

To begin entering the data, first type the name of the first terminal (“Greenbay”) in the Name field

and press Enter. The remaining fields on the screen should become active, allowing you to enter the

values listed in Table 7-1 for this terminal. First, enter a ground impedance of 0.2 + j 0.0 . Next, a


Page 7-5

current of a 1226 – j 5013 Amps should be

entered under Current Source, and 64

sections with a length of 330 m and a

tower ground resistance of 10 should be

defined under Sections (see the next

illustration).

To specify the characteristics of the

transmission lines connecting this terminal

to the central site, click on Define circuit.

The resulting screen, shown further below,

is separated vertically into two parts: the

left side is used to define the geometry of

a cross section of the line (the Conductor

Structure) and the right side is used to

define the electrical characteristics of the

static or neutral wires of the line.

The geometry of a cross section of the

transmission line is shown in Figure 2.3.

There are two static wires located at a

height of 35 m and at a distance of 7.3 m

from the center of the tower, on both sides.

To minimize the mutual interactions

between the phase wire and the static wires

and thus obtain the worst-case scenario, the

fault is assumed to occur on the phase

furthest away from the static wires, namely

Phase B2 (Phase C1 would have been just

as bad) in the figure. The coordinates of

this phase wire are X = 12 m and Y = 21.5

m. This can be specified by entering the

data shown on the screen entitled

“Conductor Specification” below.


Page 7-6

You can verify visually that the positions of

the wires are correct by clicking on the

Display button on this screen. The resulting

drawing shows a cross section of the power

line defined so far. Use the Zoom button to

look at finer details of the picture. You can

click on Illustrate to turn off the display of

the power line and return to the explanatory

illustration.

The simplest way to define the electrical

characteristics of the static wires is to import

the information from the conductor database.

To do this, click on Import from Conductor

Database. The following Category Filter

should appear.

In the Category Filter, you can narrow down

the number of conductors to be selected.

Choose Power Electric for Industry

category, All for Country, Overhead Shield

Wire for Application and Alumoweld for Conductor Type. Click Ok, the conductors that meet the

filter conditions are listed.


Page 7-7

Select “ALUMOWELD_7 No. 8, then click on Import. The data for this conductor will be exported

to the Conductor Specification screen.

This completes the data specification for Terminal Greenbay. Click OK to return to the Network

Fault Current Distribution screen.

Since the other two terminals are very similar to the

first, the simplest way to enter the corresponding data

is to create a copy of Terminal Greenbay and modify

the copy to account for the differences between the

terminals. To do this, click on Copy, enter “Hudson”

under Copy Terminal To, then click OK. We must

then correct the source current (722 – j 6453 Amps),

grid impedance (0.3 + j0.0 ) and number of sections

for this terminal (25), as well as the ground

resistances of the towers (28 ); the other

characteristics of the terminal are identical to those of

Terminal Greenbay. The remaining terminal

(“Newhaven”) can be handled in a similar way.

From the Network Fault Current Distribution screen, it is possible to view a schematic

representation of the circuit at any time by clicking on View Circuit. This invokes the GRSplits screen,

which offers a subset of the standalone utility GRSplits, which is shipped with AutoGrid Pro. You can

use the Help menu on this screen to obtain more information about the plotting options. Selecting all

three terminals, then using Plot | Plot circuit, yields the following plot, displayed in the GraRep utility.

Select File | Exit on this screen to return to the Network Fault Current Distribution screen.

The Display button on the Network Fault Current Distribution screen provides a quicker way to obtain

a plot of the circuit. When you click this button, a plot of the circuit appears directly on the screen.


Page 7-8

This plot is generated using default settings for all plotting options. You can click on Illustrate to

recover the original circuit illustration.


Page 7-9

Finally, you can click the Properties button to bring up the screen shown below. This screen allows

you to provide comments under Case Description and to select the System of Units, as was the case

for the Soil Resistivity Data Entry in Section 0.

This concludes the data entry session for the Fault Current Analysis module. Click on OK to save

your changes.

Chapter 8. Performance Evaluation of East Central Substation

Page 8-1


PERFORMANCE EVALUATION OF EAST

CENTRAL SUBSTATION

Now that we have specified an initial design for our grounding grid and that we have entered the data

defining the soil structure and the characteristics of the circuit connected to the main grounding grid,

we are ready to evaluate if our proposed design is safe and adequate.

In this chapter, we will demonstrate how to carry out the computations and how to extract the

computation results. The first step consists in determining suitable safety criteria to evaluate the

performance of the grounding grid. Then, we will show how to select a few representative reports and

graphics among those offered by the program, and how to generate those reports and graphics. These

reports and graphics will then be analyzed in order to determine at what locations, if any, mitigative

measures are required.

8.1 SAFETY CRITERIA

Before describing the steps to extract the computation results, let us first identify the safety objectives.

One of the main concerns when designing grounding systems is to ensure that no electrical hazards

exist outside or within the substation during normal and fault conditions. In most cases, there are no

safety concerns during steady-state normal conditions because very little current flows in the neutral

and grounding system. This current, called residual current, rarely exceeds 10% of the nominal load

current in most electrical distribution systems. Therefore, safety is usually a concern only during

phase-to-ground faults.

In practice, most electric substations are fenced and the fence is quite often placed 1 m (3.28 feet)

inside the outer conductor loop of the grounding system. This way, a person contacting the fence from

the outside will be standing above or close to a ground conductor which will normally result in lower

touch voltages than in the case where the fence is not surrounded by such a ground conductor loop. In

this study, the fence at the East Central Substation is located 1 m inside the outer loop of the grounding

system. Furthermore a large portion of the fence is not metallic (concrete or bricks).

Therefore, unless there are concerns for transferred potentials to remote locations via overhead or

metallic paths, such as gas, oil or water pipes, railway tracks, etc., only the area delimited by the

grounding system outer loop conductor needs to be examined with respect to unsafe touch voltages.

However, step voltages must be explored everywhere inside and outside the substation site. In general,

however, step voltages are rarely a concern inside electric power substation grounding grids, when

touch voltages have been made satisfactory; outside the grid perimeter, however, step voltages need

to be checked.


Page 8-2

8.1.1 Touch Voltages

The first safety criterion used for the evaluation of the grounding system performance is the touch

voltage limit. The touch voltage is usually defined as the difference in potential between a point on the

earth’s surface, where a person is standing, and an exposed metallic structure (present or future) within

reach of that person. Since all metallic structure within a substation should be bonded to the grounding

grid, touch voltages are calculated by computing the difference in potential between the grounding

grid and earth surface points.

ANSI/IEEE Standard 80-2000 (North America) and IEC 479-1 (Europe) provide methodologies for

determining maximum acceptable touch and step voltages, based on the minimum current required to

induce ventricular fibrillation in a human subject. The touch and step voltage limits are a function of

shock duration (i.e., fault clearing time), system characteristics (for short fault clearing times), body

weight, and foot contact resistance (which depends on the electrical resistivity of the material, such as

crushed rock or soil, on which the person is standing, its thickness, and the subsurface soil resistivity).

The table below shows how the touch voltage limit computed in accordance with ANSI/IEEE Standard

80 varies as a function of earth surface covering material, for a 0.3 s fault clearing time, a system X/R

ratio of 20, and a 50 kg body weight.

Surface Layer Touch Voltage Limit (V)

Type Resistivity (-m)

Native Soil 297 286

15 cm Crushed Rock 3000 933

The crushed rock surface layer installed on the surface of the East Central Substation is 15 cm (6)

thick and has an estimated resistivity (when wet) of 3000 -m. The maximum total clearing time of

the backup relays and circuit breakers in this example is 0.3 s. The crushed rock surface layer overlies

a soil with a resistivity of 297 -m (as will be determined in Section 8.3.3). Resistivity varies as a

function of the type of rock, the size of the stones, the moisture content and the degree of contamination

(e.g., filling of the voids between stones by finer lower resistivity material). The above table and

similar ones can be produced using the Safety module, as explained in Section 8.1.5.

8.1.2 Step Voltages

A similar table can be compiled for step voltages, defined as the difference in potential between two

points spaced 1 m (3.28 ft) apart at the earth’s surface.

Surface Layer Step Voltage Limit (V)

Type Resistivity ( -m)

Native Soil 297 558

15 cm Crushed Rock 3000 3147

Inside a substation and within 1 m (3.28 ft) outside the perimeter fence, step voltages are usually lower

than touch voltages; furthermore, the maximum acceptable values are higher than for touch voltages.


Page 8-3

Consequently, satisfying the touch voltage safety criteria in this area normally ensures that that the

step voltage safety criteria will also be satisfied. The step voltages in the substation and in an area

extending 3 m (about 10 feet) outside the substation grounding grid will be examined. Outside the

extended area of the substation, no computations will be performed. However, it is unlikely for

hazardous step voltages to exist at such remote locations when they are safe closer to the substation.

8.1.3 GPR Magnitude

Sometimes, the absolute magnitude of the GPR of the grid can be a concern. This is particularly the

case for the rating of equipment installed to isolate telecommunications lines from equipment inside

the substation. The program allows you to specify a maximum value for the GPR of the grid; a warning

is issued when the maximum GPR exceeds this value.

8.1.4 GPR Differentials

Significant potential differences between distant parts of the grounding system can give rise to local

touch voltages or equipment stress voltages when low voltage insulated conductors connect equipment

at two such locations. Appropriate protection must be in place at such locations, rated for the GPR

differentials that can arise. The GPR differentials are not going to be a concern in this study since the

grounding grid is small. If the grid is extensive or if there are buried metallic structures connected to

the grounding system, the GPR differentials should be examined. This can be done, for instance, using

the MALZ or HIFREQ Computation Modules of the CDEGS package.

8.1.5 Determining Safe Touch and Step Voltage Levels

The various parameters governing

the safety limits for touch and step

voltages can be defined in the

Safety screen. This module also

provides a quick way to specify a

surface of observation points

covering the grounding grid.

Select Project | Define Safety

Criteria to open the Safety screen

shown in the following page. As

discussed in the previous sections,

there are several parameters that

govern the magnitude of the safety

limits for the touch and step

voltages. Two of the more

important parameters can be

defined directly on the main Safety

screen, namely the Insulating

Surface Layer Resistivity and the

Fault Clearing Time. Input 0.3 as the Fault Clearing Time and 3000 as the Insulating Surface

Layer Resistivity here (the correct safety limits will be obtained later). The remaining parameters can

be defined using the Standard and Advanced safety screens. The Advanced safety screen (shown in

Click here to define

safety criteria and border

offset for observation

points


Page 8-4

the following page) allows you to define all of the safety

parameters while the Standard safety screen contains only the

most often used parameters. The Sub-Surface Uniform Soil

Layer Resistivity should be set to a representative value of the

resistivity of the soil close to the earth’s surface. Normally, this

should be equal to the resistivity of the top soil layer, although

there may be cases where you want to specify a different value

(such as when the top soil layer is very thin). Selecting the Use

top soil layer resistivity option instructs the program to always

use the resistivity of the top soil layer. When this option is

selected, the program will automatically re-compute the safety

limits whenever there are changes made to the soil model. Since

we haven’t obtained a soil model yet from the soil resistivity

measurement data, the program doesn’t know the value of the

resistivity of the top soil layer. The computed values for the touch

and step voltage limits are therefore (probably) incorrect at this

stage, but will be automatically corrected the moment the soil

resistivity analysis is complete. (If no soil data is given, the

program uses 100 -m as a default value.)

To help in selecting appropriate values for the

Fault Clearing Time and Insulating Surface

Layer Resistivity on the main Safety screen,

the Safety (Advanced) screen allows you to

examine several scenarios at once by

specifying up to 3 different fault clearing times

as well as any number of equally spaced

resistivity values for the insulating layer.

If this information is entered in the Safety

(Advanced) screen as shown here and if you

click the Get Initial Safety Values button, the Safety

Table screen shows up. This screen shows the safety

limits for touch and step voltages for all the

combinations of fault clearing times and insulating

surface layer resistivity that were specified in the

Safety (Advanced) screen.

As the screen indicates, the values shown on screen at

this point will be modified later once the sub-surface

resistivity is known.


Page 8-5

8.1.6 A Simpler Way to Specify the Location of Observation Points

As mentioned in the previous section, the main Safety screen also offers a simpler way to specify the

location of observation points for the computation of touch and step voltages. When the option

“Automatic Generation of Observation Points” is checked, the program will ignore any observation

points explicitly specified above the grid (as was done in Section 6.1) and will instead automatically

generate a rectangular surface covering the entire grid. This is shown in yellow in the screen. The

rectangular area extends beyond the region covered by the grid’s conductors by the maximum of the

amounts specified in Grid Border Offset for Touch Voltages and Grid Border Offset for Step

Voltages. By specifying 3 meters for the Grid Border Offset for Step Voltages, we are guaranteed

that the observation points will extend 3 meters outside the area of the grid, as is desired. The spacing

between the observation points can be controlled from the Automatic Observation Points Options

screen obtained by clicking on Advanced.

Moreover, the program will restrict the analysis of the touch and step voltages to the area defined by

the corresponding border offset. This means that, for the data as defined in the above screen, the step

voltages will be analyzed up to 3 meters outside the grid while the touch voltages will be analyzed up

to the edge of the grid, generally located 1 m outside the fence line.

Another advantage of using this automatic mode for the specification of the location of observation

points is that the program will automatically adjust the location of the points and of the safety analysis

areas whenever the grid is modified.

8.2 PLOTS AND REPORTS

We have now completed the data entry. What remains

is to select which reports and plots the program should

produce once the computations are complete, and to

customize the plots and reports.

8.2.1 Selecting Plots and Reports

Use Project | Report Preferences to load the Report

and Graphics Specifications screen. This screen

allows you to select which plots and reports are to be

produced once the computations are complete. These

plots and reports are produced every time the

computations are carried out.

The following reports are selected.

System Data Summary: A summary of the input

data provided to the program.

Requested Computation Reports and Plots: A

summary of the plots and reports that were requested in this screen.


Page 8-6

Resistivity Comparison: A report that lists the comparison of measured vs. computed apparent

resistivities (click the Advanced button to see this option).

List of Materials: A bill of materials report listing the characteristics of the conductors in the

grid.

Soil Resistivity Measurement Interpretation: A report showing the soil model deduced from

the provided measurements.

Ground Grid Performance: A report showing the resistance and other characteristics of the

main grounding grid and other grounding structures.

Fault Current Distribution: A report showing the results of the fault current distribution

analysis.

Safety Assessment: A report showing the safety table generated in Section 8.3.4.

Other reports are available, such as an ampacity assessment report, and a report listing the computed

self and mutual impedances of the conductors in the circuit (click the Advanced button to see the

options). They will not be generated in this tutorial.

On the Graphics tab, plots of touch and step voltage are selected, as well as a plot of the computed

and measured soil resistivities (as obtained from the Soil Resistivity analysis) and plots of various

quantities obtained from the Fault Current Distribution analysis. Other plots are available: the

Scalar Potential plot shows the earth potentials above the grid, the Grounding System

Configuration plot shows the grounding system itself and the Electric Network Configuration plot

Click on the Touch

Voltages and then

click on the View

Options…


Page 8-7

shows a schematic representation of the circuit (as was generated in Section 7.2.1). These plots have

not been requested in this tutorial.

Some of these selections can actually generate more than one plot. This, and other attributes of the

plots, can be controlled by clicking on View Options after having clicked on the pertinent item in the

window in the upper half of the screen: in this case,

click on the wording describing the option of

interest, not on the check box to the left of the

wording. For example, the options for touch voltage

plots are shown here. You can select to produce a

plot of all values of the touch voltages (Show All

Values) or a plot of only those values that are above

the safety limit for touch voltages (as defined in the

Safety screen, Section 8.3.4) or both of these plots.

For the latter plot, the regions where the touch

voltages are below the safety limits are left

transparent, making it easy to identify the

troublesome areas. Similar options are available for

step voltages. Click on Back to return to the previous

screen.

Several options are available for Fault Current

Distribution plots. You can choose to plot any

combination of the Section Span Current (i.e., the

current flowing in the shield and neutral wires along

the power lines modeled), the current discharged

into the earth by every tower and pole along the

power lines (Shunt Tower Currents) and the

ground potential rise of every tower and pole (Shunt

Tower Potentials). You can plot up to two terminals

at a time, and you can restrict the plots to a range of

towers and poles by defining the Beginning Section

and Ending Section fields (leave these fields blank

to plot all sections): recall that each “section”

represents one power line span. Note that these plots

can be helpful in gaining insight into the behavior of

the power system modeled; however, the key data

required for the design study is to be found in the

corresponding report, as will be seen.

The Type of Plots tab allows you to select the chart

types of the plots that should be produced. All the

plots selected in the Graphics tab will be produced

(if possible) using all of the selected chart types. The

2D Spot plots are generally the most useful for a

safety analysis, since they display the touch and step

voltages above the grid as a color intensity plot superposed on a plan view of the grid: these plots


Page 8-8

constitute contour plots, with the regions between the contour lines shaded in different colors to make

the voltage levels clearer.

Note that this selection of plot types applies only for some of the plots: those available under

Computation Plots apply only to plots of touch voltages, step voltages and scalar potentials, while

those under Configuration Plots apply only to plots of the grounding grid itself.

The plot and reports selection is now complete. Click OK to close this screen and return to the main

screen of AutoGrid Pro.

8.2.2 Customizing Plots

You can customize the appearance of the

plots generated by AutoGrid Pro using the

functionality of the Setup screen, available

from Project | Graphics Preferences.

In the Printer Attributes and Screen

Attributes tabs, you can select the type of

font to use in the plots, and whether color

plots should be rendered in shades of gray.

These settings can be selected independently

for printed plots and plots displayed on

screen.

The Configuration tab allows you to control

certain aspects of the plots displaying the

grounding grid, such as the scaling factors to

employ when drawing the grid, and whether or not

to show the coordinate axes on the plot.

The Computations tab can be used to customize

all other plots. You can select a display threshold

value for spot plots (or instruct the program to use

the Safe Allowable Values, as was done in this

case) and you can also set the range of the

coordinate axes for 3D perspective plots.

For greater control over the appearance of the plots, you can use the GRServer graphics processor

(available at Project | Advanced Output Processor). This program gives you full control over the

display of plots. Chapter 10 shows how to use this program in greater detail.

8.2.3 Carrying Out the Computations and Producing the Plots and Reports

Select Project | Process (or click Process on the Project Toolbar) to start the computations and create

the requested plots and reports. The program will analyze the changes you have made to the data to

determine which computations should be carried out. The following confirmation screen will appear.


Page 8-9

This screen shows which computations the program thinks it should perform. You can override the

program’s decisions by explicitly selecting or deselecting the options. You can also use this screen to

restrict the production of plots and reports to a subset of those that were selected in the Reports screen

in the previous sections.

The computations begin once you click OK on this screen. A message screen details the progress of

the computations and shows any errors or warnings generated in the run.

You can cancel the run at any time by clicking Cancel on that screen or by choosing Project | Cancel

Processing. Once the computations are complete, the requested plots and reports are produced and

displayed in the GraRep utility, as illustrated in the next section. Copies of the plots and reports are

also stored in the ‘Results’ folder that can be found in the ‘Initial Design’ sub-folder of your project

folder. The following figure shows the content of this folder after the run. The files with a “rep”

extension are the report files and those with an “EMF” extension are the plot files. The names of the


Page 8-10

report and plot files are pretty much self-explanatory. In case more details are desired, the file

“Results_Initial Design.txt” gives a short description of each file.

The following section will examine each one of these files in greater detail.

8.3 ANALYSIS OF THE RESULTS

At this stage, the GraRep (Graphics and Reports Viewer) utility contains all the plots and reports that

were produced in the computations phase. The following screen automatically appears, giving you

access to the results. The plots are stored in the View Plots tab of the utility, and the reports in its View

Reports tab.

In this section, we will examine these

reports and plots and draw conclusions as

regards the safety aspects of our initial

ground grid design.

8.3.1 Using the GraRep Utility

Before we proceed with a detailed

examination of the results, a few words

should be mentioned about the GraRep

utility. This utility displays all the

computation results produced by

AutoGrid Pro. It can be used to print these

results, both in text and graphical formats.

The following paragraphs summarize the


Page 8-11

most important features of the utility. Use GraRep’s on-line help (Help | Help Topics, or press the F1

key) to obtain more details.

GraRep displays graphical output in its View Plots tab and textual output in its View Reports tab

(there is a third tab that can be used to display messages, but is not used in AutoGrid Pro).

Several plots can be displayed in the View Plots tab, although only one can occupy the main viewing

area. The other plots can be activated by clicking on the corresponding item in GraRep’s Icon Queue,

which appears along the right-hand edge of the screen. You can select several plots simultaneously,

and print them using File | Print Selected Plots. You can also preview the printing using File | Print

Preview.

You can zoom on any part of the plot by holding the Shift key down while dragging the mouse in the

main viewing area to create a rectangle enclosing the area of interest. To restore the picture to its

original size, select Options | Fit to Size. (Do not try to magnify any part of the plot too much: at

some point the magnification process will refuse to continue, a limitation of the operating system

control used by the software.)

The View Reports tab can also hold several text reports simultaneously. The reports are all displayed

in the same window, separated by markers like Report # 1, End Report # 1. The reports can be printed

(File | Print) and previewed before printing (File | Print Preview).

8.3.2 General Information Reports

In GraRep, select the View Reports tab and scroll the window to the top (you can do this quickly by

first clicking in the window, then pressing Ctrl + Home.) The first report is a summary of the plot

and report selections for the run:

----------------------------------------------------------------

Input Data Summary Reports

----------------------------------------------------------------

System Data Summary

D:\Projects\AGP Tutorial\Initial Design\Results\System Input.rep

Requested Computation Reports and Plots

D:\Projects\AGP Tutorial\Initial Design\Results\User Input.rep

----------------------------------------------------------------

Graphics option chosen

----------------------------------------------------------------

Computation Plots

Touch Voltages

Show All Values

Show Unsafe Values Above Selected Safety Threshold

Step Voltages

Show All Values

Show Unsafe Values Above Selected Safety Threshold

Soil Resistivity Measurement Interpretation

Fault Current Distribution

Section Span Currents

Shunt Tower Currents

Shunt Tower Potentials


Page 8-12

One Terminal Plot

Terminal Number ................... 1

All Sections Selected

Configuration Plots

Grounding System Configuration

----------------------------------------------------------------

Types of plot selected

----------------------------------------------------------------

Computation Plots

2D Spot

Configuration Plots

Top View

Report #1: User Input Data

The second report summarizes the input data entered for the computations. The Safety section shows

the computed touch and step voltage safety limits.

--------------------------------------------------------------------------------------------------

-----------------------------

Project Summary

--------------------------------------------------------------------------------------------------

-----------------------------

Case Description ............................ A simple substation grounding grid analysis using

AutoGrid Pro.

Run Identification .......................... Initial Design

System of Units ............................. Metric

Radius Measured in .......................... Meters

Frequency ................................... 60 Hz

--------------------------------------------------------------------------------------------------

-----------------------------

Soil Structure (deduce soil structure from field resistivity measurements)

--------------------------------------------------------------------------------------------------

-----------------------------

Measurement method...........................Wenner

Type of measurement..........................Resistance

Probe depth option...........................Account for Probe Depth

Measurement Spacing S Apparent Depth of Depth of

Number (Meters) Resistance Current Potential

R (Ohms) Probes Do Probes Di

(Meters) (Meters)

--------------------------------------------------------------------------

R1 0.3 152.3 0.1 0.05

R2 1 48.16 0.1 0.05

R3 2 6.12 0.1 0.05

R4 5 3.34 0.1 0.05

R5 7 1.76 0.15 0.05

R6 10 1.11 0.15 0.05

R7 15 0.692 0.3 0.05

R8 25 0.441 0.3 0.05

R9 35 0.32 0.3 0.05

R10 50 0.218 0.6 0.1


Page 8-13

R11 65 0.156 0.6 0.1

R12 90 0.106 0.6 0.1

R13 120 0.079 1 0.1

R14 150 0.064 1 0.1

--------------------------------------------------------------------------------------------------

-----------------------------

Network Fault Current Distribution

--------------------------------------------------------------------------------------------------

-----------------------------

Average soil characteristics along electric lines:

Resistivity(Ohm-m) .......................... 100

Relative Permeability (p.u.) ................ 1

Central site definition:

Name ........................................ East Central

Ground Impedance (To be deduced from grounding computations)

--------------------------------------------------------------------------------------------------

-----------------------------

Safety

--------------------------------------------------------------------------------------------------

-----------------------------

Determine Safety Limits for Touch and Step Voltages

Safety Threshold for Touch Voltages ......... 933.1 V

Safety Threshold for Step Voltages .......... 3146.7 V

Automatic Generation of Observation Points.

Grid Border Offset for Touch Voltages ....... 0 m

Grid Border Offset for Step Voltages ........ 3 m

----------------------------------------------------------------

The computation results are written in the following reports:

----------------------------------------------------------------

Soil Resistivity Measurement Interpretation

D:\Projects\AGP Tutorial\Initial Design\Results\Soil Structure.rep

Ground Grid Perfomance

D:\Projects\AGP Tutorial\Initial Design\Results\Ground Grid Performance.rep

Fault Current Distribution

D:\Projects\AGP Tutorial\Initial Design\Results\Fault Current.rep

Safety Assessment

D:\Projects\AGP Tutorial\Initial Design\Results\Safety.rep

Resistivity Comparison

D:\Projects\AGP Tutorial\Initial Design\Results\Resistivity Comparison.rep

List of Materials

D:\Projects\AGP Tutorial\Initial Design\Results\Bill of Materials.rep Report #2: System Input Data

8.3.3 Soil Resistivity Analysis

The third report in GraRep summarizes the results of the soil resistivity analysis. This report shows

the computed soil structure and gives the RMS error between the computed and measured resistivities.

In our case, the soil is a two-layer soil: the top layer has a resistivity of 297 –m and a thickness of

0.67 m. The bottom layer has a resistivity of 66 –m.

=========< R E S I S T I V I T Y ( SYSTEM INFORMATION SUMMARY ) >=========

Run ID......................................: Initial Design

System of Units ............................: Meters


Page 8-14

Soil Type Selected..........................: Multi-Layer Horizontal

RMS error between measured and calculated...: 16.9464 in percent

resistivities (Note RMS=SQRT(average(Di**2)).

<--- LAYER CHARACTERISTICS --> Reflection Resistivity

Layer Resistivity Thickness Coefficient Contrast

Number (ohm-m) (Meters) (p.u.) Ratio

====== ============== ============== ============ ============

1 Infinite Infinite 0.0 1.0

2 297.0809 0.6741050 -1.0000 0.29708E-17

3 65.84766 Infinite -0.63713 0.22165

**WARNING** MORE THAN ONE SOIL MODEL CAN PRODUCE SIMILAR APPARENT RESISTIVITY

MEASUREMENT CURVES. IF YOU USE THE DEFAULT STEEPEST-DESCENT METHOD,

THEN YOU WILL MOST OFTEN OBTAIN DECENT AGREEMENT BETWEEN MEASURED

VALUES AND THE COMPUTED CURVE, WITH A REALISTIC SOIL MODEL; HOWEVER,

THE FIT MAY OCCASIONALLY BE SUB-OPTIMAL. IN SUCH CASES, THE MARQUARDT

METHOD WILL USUALLY YIELD AN EXCELLENT FIT, BUT MAY SOMETIMES SUGGEST

EXTREME RESISTIVITY VALUES. NOTE THAT DIFFERENT SOIL MODELS WILL USUALLY

YIELD SIMILAR RESULTS FOR YOUR GROUNDING SYSTEM MODELS (I.E., GPR, TOUCH &

STEP VOLTAGES), PROVIDED THAT THE GROUNDING SYSTEM IS LOCATED CLOSE TO

THE EARTH SURFACE. IF IN DOUBT, CHECK YOUR RESULTS WITH BOTH SOIL MODELS.

Report #3: Soil Resistivity Analysis

We have also requested a plot of the measured and computed resistivity values. This plot is the second

one displayed in the View Plots tab in GraRep (the first one shows the circuit, and was produced

earlier in Section 7.2.1). It shows a few measurement points that differ markedly from the computed

ones, a situation that explains the RMS error of 17% that was obtained in the run. Aside from these

discrepancies, the computed soil model fits the measured data quite well. This visual check is

important to evaluate the nature of the agreement between the measured data and the computed soil

model.

Report #4 provides a comparison of measured & computed apparent resistivities.

Comparison of Measured & Computed Apparent Resistivities

========================================================

C1-C2 SPACING APPARENT RESISTIVITY DISCREPANCY

POINT (meters) MEASURED COMPUTED Di (percent)

===== ============= ======== ======== ===========

1 0.900000 298.2 287.3 3.67

2 3.00000 303.7 179.5 40.91

3 6.00000 76.98 94.44 22.69

4 15.0000 104.9 68.16 35.05

5 21.0000 77.42 66.95 13.52

6 30.0000 69.75 66.32 4.91

7 45.0000 65.23 66.05 1.26

8 75.0000 69.28 65.93 4.83

9 105.000 70.37 65.87 6.40

10 150.000 68.49 65.85 3.86

11 195.000 63.71 65.85 3.35

12 270.000 59.94 65.85 9.85

13 360.000 59.57 65.84 10.54

14 450.000 60.32 65.84 9.14

=========

Average discrepancy: 12.14%


Page 8-15

RMS ERROR BETWEEN MEASURED AND CALCULATED RESISTIVITIES :

16.95 percent

*NOTE* RMS = SQRT( average(Di**2) )

Report #4: Soil Resistivity Comparison

8.3.4 Ground Grid Performance and Safety Analysis

The 5th report displayed in GraRep (the List of Materials report) gives some information about the

physical characteristics of the grid and its surrounding. This includes:

The quantity and size of grid conductors and rods that were used.

The number of interconnections in the grid at which bonding will be required.

The characteristics of the insulating layer (surface area, thickness, volume, resistivity)


Page 8-16

****************************************************************

List of Materials

Creation Date/Time: 7 Jan 2012/13:23:26

****************************************************************

Interconnection / Bonding Nodes ....................... 63

Extent of Grounding System ............................ 6000 (Square Meters)

Surface Layer Thickness ............................... 15 (Centimeters)

Volume of Insulating Layer ............................ 900 (Cubic meters)

Wet Resistivity of Insulating Surface Layer ........... 3000 (Ohm-m)

Grounding System Data

Number of Rods Length (m) Diameter (m)

----------------------------------------------------------------

None - -

Number of Grid Conductors Length (m) Diameter (m)

----------------------------------------------------------------

7 100 0.012

9 60 0.012

Total Length of Grid Conductors (m) Diameter (m)

----------------------------------------------------------------

1240 0.012

Report #5: List of Materials

The next report shows a table of computed safety limits corresponding to different values of resistivity

of the insulating layer and different fault clearing times. Note the “Equivalent Sub-Surface Layer

Resistivity” entry. The value used here is the value that was computed for the resistivity of the top soil

layer, as shown in the previous section. The remaining data reflects the data we entered in Section

8.1.5.

Report #6:

Date of run (Start) = Saturday,07 January 2012

Starting Time = 1:23:26 PM

>>Safety Calculation Table

System Frequency....................................: 60.000(Hertz)

System X/R..........................................: 20.000

Surface Layer Thickness.............................: 15.000(cm)

Number of Surface Layer Resistivities...............: 10

Starting Surface Layer Resistivity..................: NONE

Incremental Surface Layer Resistivity...............: 500.00(ohm-m)

Equivalent Sub-Surface Layer Resistivity........... .: 297.08(ohm-m)

Body Resistance Calculation.........................: IEEE Std.80-2000

Fibrillation Current Calculation....................: IEEE Std.80-2000 (50kg)

Foot Resistance Calculation.........................: IEEE Std.80-2000

User Defined Extra Foot Resistance..................: 0.0000 ohms

==============================================================================

Fault Clearing Time (sec) | 0.100 | 0.200 | 0.300 |

+----------------------------+---------------+---------------+---------------+

Decrement Factor | 1.232 | 1.125 | 1.085 |

Fibrillation Current (amps)| 0.367 | 0.259 | 0.212 |


Page 8-17

Body Resistance (ohms)| 1000.00 | 1000.00 | 1000.00 |

==============================================================================

==========================================================================

| Fault Clearing Time | |

Surface |-----------------+-----------------+-----------------| Foot |

Layer | 0.100 sec. | 0.200 sec. | 0.300 sec. | Resist |

Resist |-----------------|-----------------|-----------------| ance |

ivity | Step | Touch | Step | Touch | Step | Touch | 1 Foot |

(ohm-m) |Voltage |Voltage |Voltage |Voltage |Voltage |Voltage | (ohms) |

|(Volts) |(Volts) |(Volts) |(Volts) |(Volts) |(Volts) | |

==========================================================================

NONE | 850.5| 435.9| 658.8| 337.7| 557.7| 285.8| 928.4|

|---------+--------+--------+--------+--------+--------+--------+--------+

500.0| 1159.3| 513.1| 898.1| 397.5| 760.3| 336.5| 1447.1|

|---------+--------+--------+--------+--------+--------+--------+--------+

1000.0| 1896.9| 697.5| 1469.4| 540.3| 1244.0| 457.4| 2685.9|

|---------+--------+--------+--------+--------+--------+--------+--------+

1500.0| 2625.2| 879.6| 2033.6| 681.4| 1721.6| 576.8| 3909.2|

|---------+--------+--------+--------+--------+--------+--------+--------+

2000.0| 3350.7| 1060.9| 2595.5| 821.8| 2197.3| 695.7| 5127.6|

|---------+--------+--------+--------+--------+--------+--------+--------+

2500.0| 4074.9| 1242.0| 3156.5| 962.1| 2672.2| 814.5| 6343.9|

|---------+--------+--------+--------+--------+--------+--------+--------+

3000.0| 4798.4| 1422.9| 3717.0| 1102.2| 3146.7| 933.1| 7559.0|

|---------+--------+--------+--------+--------+--------+--------+--------+

3500.0| 5521.5| 1603.7| 4277.1| 1242.2| 3620.9| 1051.6| 8773.6|

|---------+--------+--------+--------+--------+--------+--------+--------+

4000.0| 6244.4| 1784.4| 4837.1| 1382.2| 4094.9| 1170.2| 9987.6|

|---------+--------+--------+--------+--------+--------+--------+--------+

4500.0| 6967.1| 1965.0| 5396.9| 1522.2| 4568.9| 1288.6| 11201.4|

|---------+--------+--------+--------+--------+--------+--------+--------+

* Note * Listed values account for short duration asymmetric waveform

decrement factor listed at the top of each column. Report #6: Safety Limits Calculation Table

This report depicts the safety threshold values applicable to various scenarios of clearing times and

surface layer resistivities. It indicates that touch voltages of 933 V or less and step voltages of 3147 V

or less are safe if a 15 cm (6) crushed rock layer with a resistivity of 3000 -m is overlying a native

soil with a resistivity of 297 -m, for a 0.3 s fault clearing time. Touch voltages of 286 V or less and

step voltages of 558 V or less are safe if no surface crushed rock is present at East Central Substation.

Note that outside the substation where there is no crushed rock, step voltages must not exceed 558

Volts.

The next report summarizes the performance aspects of the grounding grid.

DATE OF RUN (Start)= DAY 7 / Month 1 / Year 2012

STARTING TIME= 13:23:26:67

===========< G R O U N D I N G ( SYSTEM INFORMATION SUMMARY ) >===========

Run ID......................................: Initial Design

System of Units ............................: Metric

Earth Potential Calculations................: Single Electrode Case

Type of Electrodes Considered...............: Main Electrode ONLY

Soil Type Selected..........................: Multi-Layer Horizontal

SPLITS/FCDIST Scaling Factor................: 9.3560

1

1


Page 8-18

MULTI-LAYER EARTH CHARACTERISTICS USED BY PROGRAM

LAYER TYPE REFLECTION RESISTIVITY THICKNESS

No. COEFFICIENT (ohm-meter) (METERS)

----- ------ ------------- ------------- -------------

1 Air 0.00000 0.100000E+21 Infinite

2 Soil -0.999990 297.081 0.674105

3 Soil -0.637132 65.8477 Infinite

1

CONFIGURATION OF MAIN ELECTRODE

===============================

Original Electrical Current Flowing In Electrode..: 1000.0 amperes

Current Scaling Factor (SPLITS/FCDIST/specified)..: 9.3560

Adjusted Electrical Current Flowing In Electrode..: 9356.0 amperes

Number of Conductors in Electrode.................: 16

Resistance of Electrode System....................: 0.53773 ohms

SUBDIVISION

===========

Grand Total of Conductors After Subdivision.: 110

Total Current Flowing In Main Electrode......: 9356.0 amperes

Total Buried Length of Main Electrode........: 1240.0 meters

EARTH POTENTIAL COMPUTATIONS

============================

Main Electrode Potential Rise (GPR).....: 5031.0 volts

Report #7: Grounding Grid Performance

This report shows that the ground resistance of the East Central substation grid is 0.538 , and that

the fault current injected into the grid is 9.36 kA, for a ground potential rise (GPR) of 5.03 kV.

We also requested 5 plots related to the performance of the grounding grid. The first one (and the next

in the queue in GraRep’s View Plots tab) is a plan view of the grounding grid itself.

The following plot shows the touch voltages throughout the grid. Recall from Section 8.1 that the

touch voltages are computed up to a distance of 1 meter outside the fence line, i.e. up to the edge of

the grid. The touch voltages can reach values as large as 2.28 kV. The larger values occur in the corners

of the grid, which is typical. These values are very much above the safety limit for touch voltages (933

Volts) that was displayed in the safety report.


Page 8-19

This can be verified in the next plot, which shows the “unsafe” values of the touch voltages. In this

plot, any value of the touch voltages falling below the 933 Volts limit is left transparent; therefore all

colored areas are above the limit and should be considered unsafe. Most of the plot is colored for this

initial design: we will therefore have to reinforce the grid substantially to eliminate this problem.


Page 8-20

The next plot shows the step voltages over an area extending up to 3 meters outside the grid. The

maximum step voltage reached is 525 Volts. This is below the safe step voltage limit computed in the

presence of crushed rock (3147 Volts). This can be verified in the step voltage plot following the first

one: only “unsafe step voltages” are colored. Since there are no colored areas over the grid, the grid is

completely safe from the point of view of step voltages. Note that the step voltages are also safe even

when there isn’t any crushed rock, since in this case the safety limit for step voltages is 558 Volts.

The situation encountered here is quite common: the touch voltages over the grid are unsafe but the

step voltages are fine. The safety requirements for step voltages are usually easier to meet than those

for touch voltages.

It is important to mention that computations of step voltages are sensitive to the location of observation

points. When the maximum computed step voltage is on the border line as compared with the safe step

voltage limit (525 V vs. 558 V for native soil in this case), it is recommended to reduce the spacing

between observation points to less than 1 m in order to capture the worst case step voltage which are

usually at the corners of a substation where earth potentials drop quickly.


Page 8-21

8.3.5 Computation of Fault Current Distribution

The next report in GraRep’s View Reports tab summarizes the results of the fault current distribution

analysis. This report shows the computed value of the current injected into the East Central substation

grid (the “Total Earth Current”) as well as a wealth of information regarding the various terminals of

the circuit. Note that the computations used the computed value of the grid’s “Ground Resistance”,

namely 0.538 .

DATE OF RUN (Start)= DAY 7 / Month 1 / Year 2012

STARTING TIME= 13:23:26:69


Page 8-22

=======< FAULT CURRENT DISTRIBUTION ( SYSTEM INFORMATION SUMMARY ) >=======

Run ID.......................................: Initial Design

Central Station Name.........................: East Central

Total Number of Terminals....................: 3

Average Soil Resistivity.....................: 100.00 ohm-meters

Printout Option..............................: Detailed

1

Central Station: East Central

Ground Resistance........................: 0.53773 ohms

Ground Reactance.........................: 0.0000 ohms

1

Terminal No. 1 : Greenbay

Number of Sections..............: 64

Ground Impedance................: 0.20000 +j 0.0000 ohms

Source Current..................: 5160.7 Amps / -76.257 degrees

Neutral Connection Impedance....: 0.0000 +j 0.0000 ohms

Span Length.....................: 330.00 m

1

Terminal No. 2 : Hudson





Span Length.....................: 330.00 m

1

Terminal No. 3 : NewHaven





Span Length.....................: 330.00 m

1

TERMINAL GROUND SYSTEM (Magn./Angle)

Term: 1 Total Earth Current...: 3869.0 Amps / 93.292 deg.

Earth Potential Rise..: 773.80 Volts / 93.292 deg.





Average Resistivity...........: 100.00 Ohm-meters

Grid Impedance................: 0.53773 +j 0.0000 Ohms

< Magnitude / Angle >

Total Fault Current...........: 17355. Amps / -81.073 degrees

Total Neutral Current.........: 8068.5 Amps / -75.565 degrees

Total Earth Current...........: 9356.0 Amps / -85.821 degrees

Ground Potential Rise (GPR)...: 5031.0 Volts / -85.821 degrees

Report #8: Fault Current Distribution Calculation

The next plot on the View Plot tab of GraRep shows the “Section Current” (i.e., the current flowing

in the shield wires of the transmission line) for the first terminal of the circuit. The plot shows that a

considerable portion of the fault current (about 3000 A) leaves the fault site via the shield wires. This

current, however, quickly flows into the earth via the first few towers and then levels off to a constant

“trapped” current maintained by induction from the faulted phase.


Page 8-23

The next plot shows the “Shunt Current”, i.e. the current flowing in every tower structure of the

transmission line. It confirms that most of the current is discharged close to the fault site: we see that

the values drop off quite abruptly and then increase again as the terminal station is approached.

This is also reflected in the following plot (“Shunt Potential”), which shows the GPR of the tower

structures along the transmission line.


Page 8-24

Chapter 9. Reinforcing The Grounding System

Page 9-1


REINFORCING THE GROUNDING SYSTEM

The touch voltages obtained in Chapter 8 indicate that our initial design is quite far from providing a

safe ground grid design: touch voltages exceed the safe limit at most locations throughout the

substation. The highest values occur in the corner meshes of the grounding grid, which suggests that

there is a need to have more conductors towards the edge of the grounding system than towards the

central portion. This observation is consistent with analytical and experimental results. However, the

optimum or most efficient conductor compaction at the periphery of a grounding system depends on

many factors, particularly on earth structure characteristics. Moreover, practical considerations often

introduce additional constraints, which must be accounted for. In general, however, the following

crude rules of thumb can be used as a preliminary set of guidelines:

When the surface (shallow depth) soil resistivity is small compared to that of the deeper layers

(those which are not in contact with the grounding system), use grids with more conductors at

the edge than in the central area (exponentially-spaced conductors). The degree of conductor

clustering (compactness) at the periphery of the grid should increase with an increase in the

contrast between the surface and deep layer resistivities.

When the surface soil resistivity becomes larger than that of the deeper soil layers, the

clustering (compactness) ratio should decrease towards a uniform distribution of conductors

in the case where the contrast ratio is significant (5 or more) and the thickness of surface layers

is small compared to the size of the grounding system (1/5 or less).

Finally, when the surface soil resistivity is quite large compared to that of the deeper layers

and its thickness is small enough so that use of ground rods penetrating into the deeper layer

is efficient, a number of ground rods should be installed wherever possible to reduce the GPR,

touch and step voltages instead of using unequally spaced conductors.

Based on the soil model and the initial design, we will combine the first and the third methods in this

study. We will also increase the total number of conductors in the grid. The improvements will be

carried out in two steps: first, the grid itself will be modified to use a denser exponential design, then

some grounding rods will be added.

9.1 EXPONENTIAL GRID DESIGN

At this point, we want to make some modifications to the grounding grid. We could modify the data

directly in the “Initial Design” scenario we have been working with so far. If we do this, the “Initial

Design” scenario will be lost. Instead, we will use a different scenario to make the changes. If you

have chosen to enter the data manually, proceed to the next section to create a new scenario. Otherwise,

go to Section 9.1.3 that shows how to open an existing scenario.


Page 9-2

9.1.1 Creating the Exponential Grid Scenario

Select Project | New Scenario. In the

resulting dialog, specify “Exponential

Grid” for the Scenario Name. The

program will automatically define a

Scenario File Location, which you can

override. Make sure to select Based On

Existing Scenario for the Reference

Scenario and to use Initial Design as a

reference.

Click Create on the New Scenario screen.

This instructs the program to create a copy

of the Initial Design scenario and give it the name “Exponential Grid”. You can now proceed to Section

9.1.3 to make the necessary changes to this scenario.

9.1.2 Opening the Exponential Grid Scenario

To open an existing scenario, select

Project | Open Scenario. Make sure that

the Existing tab is active, then select the

Exponential Grid scenario from the list.

Click Open to open this scenario.

9.1.3 Modifying the Exponential Grid Scenario

At this point, your AutoGrid Pro screen should look as follows. The Active Scenario, that is, the

scenario that can presently be edited, is Exponential Grid. The other scenario (Initial Design) will no

longer be used in this tutorial and can be closed by closing the window containing the drawing of the

grid (the one that shows Scenario Initial Design in its title bar).


Page 9-3

The simplest way to modify the grid is to use the Edit | Edit Object command. To do this, the grid

should first be selected. To select the grid, click on any point on the grid; the grid should turn red to

confirm your selection. This operation will be easier to perform if you first turn off the display of the

observation profiles. To do this, uncheck the option View | Profiles.

Once the grid is selected, select Edit | Edit Object. The screen should appear in the following page.

This screen is very similar to the Create Object screen used in Section 6.1 and is used in the same way.

Make the following changes:

Parameter Old Value New Value

Nab 7 13

Nac 9 17

Compression Ratio 1 0.8


Page 9-4

We have increased the number of conductors substantially and defined a compression ratio different

than 1. This last option instructs the program to bunch the conductors located towards the edge of the

grid more closely together. The ratio of the distance between successive pairs of conductors decreases

by the factor entered in this field.

The resulting screen is shown in the following page. Click OK to confirm the changes. The modified

grid will be shown in the main drawing.

Since we know that the step voltages are safe already, we will turn off the production of the step

voltage plots. Select Project | Define Safety Criteria and uncheck the Step Voltages option under

Determine Safety For. Click OK in the Safety screen to return to the main screen. (If the safety limits

are not correct, they will be updated after computations.)


Page 9-5

The data entry is complete. Select Project | Process to start the

computations and click OK on the confirmation screen. The

program will automatically compute the grid impedance, the fault

current and the touch voltages.

Once the computations are complete, the analysis of the results can

proceed as in Section 8.3. Here, we will focus only on the touch

voltages, since we know them to be problematic.

The results are shown in the following page. The maximum touch

voltage has been reduced to 1.15 kV, which is still above the safety

limit. The plot of unsafe touch voltages shows that the large values

of touch voltages are concentrated at the periphery of the grid. It is

therefore likely that adding ground rods at the corners of the grid

will fix this problem. This is what we will attempt in the next

section.


Page 9-6

9.2 ADDING GROUND RODS

As noted above, adding ground rods to the grid should fix the last problem with the touch voltages.

We will briefly describe how to do this in this section. The steps are very similar to what was done in

the previous section, namely create a new scenario based on the current one, modify the data by adding

rods, run it and analyze the results.

9.2.1 The Details

If you are entering the data manually, create a new scenario as was done in Section 9.1.1,

using the name Exponential Grid With Rods for the scenario and using the Exponential

Grid scenario as a reference. If you are following the pre-made tutorial, then simply open

the existing Exponential Grid With Rods scenario as was done in Section 9.1.2.

To add rods to the corners of the grid, select Options | Pointer Mode | Power Tool, or simply click

on the button on the toolbar at the left of the main screen. This will load the Power Tool.

Make sure to select the Create Rod option. By default, the program will create 10 meter long rods

with a radius of 0.01 meters. We will keep these default settings, but will change the length from 10

m to 20 m.


Page 9-7

With the Power Tool still

loaded, click successively

at the four corners of the

grid. As you do this, the

picture should change to

show the newly created rod

as an empty circle. It

should also indicate with

magenta circles the

location of the nodes in the

grid that are connected to

the one you clicked. This

makes it easier to verify

that the desired point was

clicked on, and not a

neighboring one. You

don’t have to be very

precise with the mouse

clicks, since the program

will automatically snap to

the closest conductor. In fact, if you don’t click close enough to a conductor, the program will refuse

to create a rod and warn you about it. Note also that you can easily undo any undesired action using

Edit / Undo.

To close the Power Tool, select any other option under Options | Pointer Mode, typically Select

Objects.

Once all four rods are created, select Project | Process. Once the computations are complete, we can

look at the touch voltage plot. This time, all values are safe, the maximum value being 874 Volts, and

the analysis is complete.

Further design iterations may be required to remove or reposition some conductors to more practical

locations. Extra ground rods may be added to account for winter soil freezing or summer extreme

drought conditions. In some cases, other soil structure models may need to be analyzed to account for

inherent data uncertainties or known soil characteristic variations. In such cases, the worst-case

scenario should be retained as a reference for the final recommended grounding design configuration.


Page 9-8

9.3 EXPORT GROUNDING GRID INTO DXF FILE

At this stage, it is possible to send directly the grid configuration to a DXF compatible CAD drawing

system. For example if your CAD system is AUTOCAD (or DXF compatible), you may proceed as

follows:

In the AutoGrid Pro, select Save Document As… under the Files menu. Select the CAD Files from

the Files of types, change the file name to “Exponential Grid With Rods.DXF” and click on OK. The

file “Exponential Grid With Rods.DXF” is created.

On a final note, it is worthwhile mentioning that when redesigning an existing grounding system

(update, upgrade, etc.), you could import the actual system configuration from a DXF-compatible

CAD file by clicking the Import… under the Files menu in the AutoGrid Pro (after a New Document

is created in AutoGrid Pro). It is however important to note here that some drawings may contain

overlapping lines which will ultimately result in invalid overlapping conductors in MALT.

Furthermore, too many details such as short wire connections and bonding conductors have a minimal


Page 9-9

impact on the grounding design performance but a significant negative impact on the computations in

terms of run time and run accuracy. One way to remove this kind of problem is to use the minimum

conductor length threshold to ignore such non-significant short conductors. This strategy, however,

can be used in MALT only and may have a negative impact on the node subdivision process.

Chapter 10. Using GRServer

Page 10-1


USING GRSERVER

This chapter shows how to use the GRServer program to examine the computation results of AutoGrid

Pro in greater detail. This program allows you to produce high-quality 2D and 3D plots of the results,

and to save those plots to disk or print them. It also allows you to manipulate the plots (e.g., rotate,

translate and scale them) for better viewing. While this program is somewhat more difficult to use than

the GraRep utility (which is used by default in AutoGrid Pro), the greater quality of the graphics it

generates may well be worth the extra effort.

This chapter is not an essential part of the tutorial, and can be skipped if you are not interested in

creating plots of higher quality.

10.1 STARTING GRSERVER

To start the GRServer program, use Project | Advanced Output Processor or click on the GRServer

button on the Project toolbar in AutoGrid Pro. This starts the program, and loads the computation

databases for the active scenario in AutoGrid Pro. Note that these computation databases, which are

created when processing a scenario, should be available before starting GRServer. This is the case

here, since we created the databases for scenario Exponential Grid With Rods in the previous step.

The screen shown in the following page should appear when the program first loads. The main screen

of the program displays an empty plot window, and three options (Soil, Grid and Circuit) are available

in the toolbar to the left of the screen, corresponding to the computation options in AutoGrid Pro.

These options are available as the first, second and last buttons in the toolbar. The other buttons in that

toolbar allow you to create plots for the MALZ, HIFREQ and SPLITS programs of the CDEGS

package; these are not available in AutoGrid Pro.

The Plot Options window should also be visible. This window is the main interface to create and

customize plots in GRServer. This window may disappear during the execution the program. If this

happens, you can use Plot | Options to bring it back.


Page 10-2

The program is presently ready to create soil

resistivity plots. Click on the Grid button

(the second button from the top) on the

toolbar to activate the grid plotting module,

which is used to plot touch and step voltages.

A second blank plot should appear.

In the Plot Options window, click on the

More (>>) button to open the Computations

Setup screen. Select Touch Voltages under

Determine. You can then collapse this

screen by clicking on the Less (<<) button in

the Plot Options window. At this point, we

are ready to create touch voltages plots using

GRServer.

10.2 CREATING 3D PLOTS

By default, the program creates a 3D perspective plot of the touch voltages. Click on Draw in the Plot

Options window to generate the plot. The screen should look as follows (the touch voltage plot

window was maximized, for easier viewing).


Page 10-3

Note that the maximum touch voltage displayed on the plot is about 1.92 kV, as opposed to 929 V

when plotted directly with AutoGrid Pro. The reason for this difference is that AutoGrid Pro restricts

the computation of the touch voltages to a region just covering the grid. This is explained in Section

8.1.6, where the Grid Border Offset for Touch Voltages is specified as 0. This information is not

transmitted to GrServer, which examines the touch voltages over the entire area covered by

observation points, namely up to 3 meters outside the grid.

You can restrict the analysis of the touch voltages to points that are located only above the grid using

the Zoom Polygon feature of GrServer. This feature allows you to restrict the display of computed

quantities (touch or step voltages) to points that lie inside a specified polygon. To restrict the analysis

to points located directly above the grid, the polygon should be a rectangle of the same size as the grid

(60 m by 100 m), with one corner at the origin of the coordinate system.

To specify this zoom polygon, first click on the More (>>) button in the Plot Options window to open

the Computations Setup screen, and select the Zoom & Report tab (on the left side of the window).

Enter the following numbers in the Search Zone Vertices Table:

No X Pos Y Pos Z Pos

1 0 0 0

2 0 60 0

3 100 60 0

4 100 0 0

Finally, click Draw. The following plot should be produced. This plot agrees with the result obtained

previously with AutoGrid Pro (Section 9.2.1).


Page 10-4

Several aspects of the plot can be customized. For example, the plot can be easily rotated. To do that,

select Plot | Rotate | Free or click on the last button in the program’s main toolbar, and select the Free

option. Then, drag the mouse (i.e., move the mouse while the mouse button is pressed) in the plot area:

a cube indicating the new position of the plot follows the movements of the mouse. Once you release

the mouse button, the plot is redrawn in the new position.

There are many other options to control the rotation of the plots. For instance, you can restrict the

rotation to be around one the axes of the coordinate system. You can also move the plot (Plot | Move),

scale it up and down (Plot | Scale) or zoom on any region of interest in the plot (Plot | Zoom).

Other options are available to control the appearance

of the plots. These are regrouped in the 3D Advanced

Plot Setup window. Click on the Plots button in the

Plot Options window to get to that screen. You can

control the color of certain plot elements, whether or

not the legend is displayed, etc… For example, the

following figure shows what happens when a “Spot”

ceiling is requested by selecting the Spot option

under Ceiling Projection Types.


Page 10-5

10.3 CREATING 2D PLOTS

The GRServer program can also

be used to create 2D plots. To do

this, select 2D under Plot View

in the Plot Options window,

select Touch Voltages, and

click on Draw. (Note that the

plot below was generated with

the Zoom Polygon option turned

off.)

When moving the mouse cursor

in the resulting plot, the value of

the X coordinate (here, the

distance from the starting point

of the computation profile) and

of the Y coordinate (here, the

touch voltage at that point)

corresponding to the location of

the mouse in the plot are shown

in the program’s status bar.

10.4 SAVING AND PRINTING PLOTS

You can save a plot produced in GRServer by selecting File | Save As when that plot is the active plot.

The following screen should appear. By default, the program offers to save the file as ‘mt_Exponential

Grid With Rods.emf’ in the Exponential Grid With Rods scenario folder. If this isn’t satisfactory, you


Page 10-6

can click on Save As again, then browse to the desired filename. Click OK in the Save As screen to

complete the operation.

You can also print the current plot, or even print all open plots or only the selected plots. To print the

currently selected plots, use File | Print (or File | Print Selected Plots if more than one plot is currently

selected) and follow the instructions in the ensuing dialogs. To print all the plots, select File | Print

All Plots.

10.5 SUMMARY

This chapter has described briefly the main features of the GRServer plotting program. The capabilities

of the program were illustrated by producing 2D and 3D plots of the touch voltage above the grounding

grid studied in scenario Exponential Grid With Rods. The program could also be used to generate the

soil resistivity plots and the fault current analysis plots for that scenario (or any AutoGrid Pro

scenario).

Only a few of the program’s options were explored. Consult GRServer’s on-line help for more

details on the options available in that program.

To exit the GRServer program, select File | Exit.

Chapter 11. Conclusion

Page 11-1


CONCLUSION

This concludes our concise step-by-step instructions on how to prepare, submit and examine results

for a simple grounding analysis problem using AutoGrid Pro. You can use File | Exit to quit the

program. Accept to save the changes when prompted to do so.

Only a few of the many features of the software have been used in this tutorial. You should try the

many other options available to familiarize yourself with the CDEGS software package. Your SES

Software distribution medium also contains a wealth of information stored under the PDF directory.

There you will find the Getting Started with SES Software Packages manual (\PDF\getstart.pdf)

which contains useful information on the CDEGS environment. You will also find other How

To…Engineering Guides, Annual Users’ Group Meeting Proceedings and much more. All Help

documents are also available online.

Notes

NOTES

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